[PATCH] exec: Make sure task->comm is always NUL-terminated

[PATCH] exec: Make sure task->comm is always NUL-terminated

[Kees Cook]

Using strscpy() meant that the final character in task->comm may be
non-NUL for a moment before the "string too long" truncation happens.

Instead of adding a new use of the ambiguous strncpy(), we'd want to
use memtostr_pad() which enforces being able to check at compile time
that sizes are sensible, but this requires being able to see string
buffer lengths. Instead of trying to inline __set_task_comm() (which
needs to call trace and perf functions), just open-code it. But to
make sure we're always safe, add compile-time checking like we already
do for get_task_comm().

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: linux-mm@kvack.org
Cc: linux-fsdevel@vger.kernel.org

Here's what I'd prefer to use to clean up set_task_comm(). I merged
Linus and Eric's suggestions and open-coded memtostr_pad().
---
 fs/exec.c             | 12 ++++++------
 include/linux/sched.h |  9 ++++-----
 io_uring/io-wq.c      |  2 +-
 io_uring/sqpoll.c     |  2 +-
 kernel/kthread.c      |  3 ++-
 5 files changed, 14 insertions(+), 14 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index e0435b31a811..5f16500ac325 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1200,16 +1200,16 @@ char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
 EXPORT_SYMBOL_GPL(__get_task_comm);
 
 /*
- * These functions flushes out all traces of the currently running executable
- * so that a new one can be started
+ * This is unlocked -- the string will always be NUL-terminated, but
+ * may show overlapping contents if racing concurrent reads.
  */
-
 void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
 {
-	task_lock(tsk);
+	size_t len = min(strlen(buf), sizeof(tsk->comm) - 1);
+
 	trace_task_rename(tsk, buf);
-	strscpy_pad(tsk->comm, buf, sizeof(tsk->comm));
-	task_unlock(tsk);
+	memcpy(tsk->comm, buf, len);
+	memset(&tsk->comm[len], 0, sizeof(tsk->comm) - len);
 	perf_event_comm(tsk, exec);
 }
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index e6ee4258169a..ac9f429ddc17 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1932,11 +1932,10 @@ static inline void kick_process(struct task_struct *tsk) { }
 #endif
 
 extern void __set_task_comm(struct task_struct *tsk, const char *from, bool exec);
-
-static inline void set_task_comm(struct task_struct *tsk, const char *from)
-{
-	__set_task_comm(tsk, from, false);
-}
+#define set_task_comm(tsk, from) ({			\
+	BUILD_BUG_ON(sizeof(from) != TASK_COMM_LEN);	\
+	__set_task_comm(tsk, from, false);		\
+})
 
 extern char *__get_task_comm(char *to, size_t len, struct task_struct *tsk);
 #define get_task_comm(buf, tsk) ({			\
diff --git a/io_uring/io-wq.c b/io_uring/io-wq.c
index a38f36b68060..5d0928f37471 100644
--- a/io_uring/io-wq.c
+++ b/io_uring/io-wq.c
@@ -634,7 +634,7 @@ static int io_wq_worker(void *data)
 	struct io_wq_acct *acct = io_wq_get_acct(worker);
 	struct io_wq *wq = worker->wq;
 	bool exit_mask = false, last_timeout = false;
-	char buf[TASK_COMM_LEN];
+	char buf[TASK_COMM_LEN] = {};
 
 	set_mask_bits(&worker->flags, 0,
 		      BIT(IO_WORKER_F_UP) | BIT(IO_WORKER_F_RUNNING));
diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c
index a26593979887..90011f06c7fb 100644
--- a/io_uring/sqpoll.c
+++ b/io_uring/sqpoll.c
@@ -271,7 +271,7 @@ static int io_sq_thread(void *data)
 	struct io_ring_ctx *ctx;
 	struct rusage start;
 	unsigned long timeout = 0;
-	char buf[TASK_COMM_LEN];
+	char buf[TASK_COMM_LEN] = {};
 	DEFINE_WAIT(wait);
 
 	/* offload context creation failed, just exit */
diff --git a/kernel/kthread.c b/kernel/kthread.c
index db4ceb0f503c..162d55811744 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -736,10 +736,11 @@ EXPORT_SYMBOL(kthread_stop_put);
 
 int kthreadd(void *unused)
 {
+	static const char comm[TASK_COMM_LEN] = "kthreadd";
 	struct task_struct *tsk = current;
 
 	/* Setup a clean context for our children to inherit. */
-	set_task_comm(tsk, "kthreadd");
+	set_task_comm(tsk, comm);
 	ignore_signals(tsk);
 	set_cpus_allowed_ptr(tsk, housekeeping_cpumask(HK_TYPE_KTHREAD));
 	set_mems_allowed(node_states[N_MEMORY]);
-- 
2.34.1

Re: [PATCH] exec: Make sure task->comm is always NUL-terminated

[Linus Torvalds]

[-- Attachment #1: Type: text/plain, Size: 1706 bytes --]

Edited down to just the end result:

On Fri, 29 Nov 2024 at 20:49, Kees Cook <kees@kernel.org> wrote:
>
>  void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
>  {
>         size_t len = min(strlen(buf), sizeof(tsk->comm) - 1);
>
>         trace_task_rename(tsk, buf);
>         memcpy(tsk->comm, buf, len);
>         memset(&tsk->comm[len], 0, sizeof(tsk->comm) - len);
>         perf_event_comm(tsk, exec);
>  }

I actually don't think that's super-safe either. Yeah, it works in
practice, and the last byte is certainly always going to be 0, but it
might not be reliably padded.

Why? It walks over the source twice. First at strlen() time, then at
memcpy. So if the source isn't stable, the end result might have odd
results with NUL characters in the middle.

And strscpy() really was *supposed* to be safe even in this case, and
I thought it was until I looked closer.

But I think strscpy() can be saved.

Something (UNTESTED!) like the attached I think does the right thing.
I added a couple of "READ_ONCE()" things to make it really super-clear
that strscpy() reads the source exactly once, and to not allow any
compiler re-materialization of the reads (although I think that when I
asked people, it turns out neither gcc nor clang rematerialize memory
accesses, so that READ_ONCE is likely more a documentation ad
theoretical thing than a real thing).

And yes, we could make the word-at-a-time case also know about masking
the last word, but it's kind of annoying and depends on byte ordering.

Hmm? I don't think your version is wrong, but I also think we'd be
better off making our 'strscpy()' infrastructure explicitly safe wrt
unstable source strings.

          Linus

[-- Attachment #2: patch.diff --]
[-- Type: text/x-patch, Size: 1168 bytes --]

 lib/string.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/lib/string.c b/lib/string.c
index 76327b51e36f..a2a678e45389 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -137,7 +137,7 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count)
 	if (IS_ENABLED(CONFIG_KMSAN))
 		max = 0;
 
-	while (max >= sizeof(unsigned long)) {
+	while (max > sizeof(unsigned long)) {
 		unsigned long c, data;
 
 		c = read_word_at_a_time(src+res);
@@ -153,10 +153,10 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count)
 		max -= sizeof(unsigned long);
 	}
 
-	while (count) {
+	while (count > 0) {
 		char c;
 
-		c = src[res];
+		c = READ_ONCE(src[res]);
 		dest[res] = c;
 		if (!c)
 			return res;
@@ -164,11 +164,11 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count)
 		count--;
 	}
 
-	/* Hit buffer length without finding a NUL; force NUL-termination. */
-	if (res)
-		dest[res-1] = '\0';
+	/* Final byte - force NUL termination */
+	dest[res] = 0;
 
-	return -E2BIG;
+	/* Return -E2BIG if the source continued.. */
+	return READ_ONCE(src[res]) ? -E2BIG : res;
 }
 EXPORT_SYMBOL(sized_strscpy);
 

Re: [PATCH] exec: Make sure task->comm is always NUL-terminated

[Kees Cook]

On Fri, Nov 29, 2024 at 11:15:44PM -0800, Linus Torvalds wrote:
> Edited down to just the end result:
> 
> On Fri, 29 Nov 2024 at 20:49, Kees Cook <kees@kernel.org> wrote:
> >
> >  void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
> >  {
> >         size_t len = min(strlen(buf), sizeof(tsk->comm) - 1);
> >
> >         trace_task_rename(tsk, buf);
> >         memcpy(tsk->comm, buf, len);
> >         memset(&tsk->comm[len], 0, sizeof(tsk->comm) - len);
> >         perf_event_comm(tsk, exec);
> >  }
> 
> I actually don't think that's super-safe either. Yeah, it works in
> practice, and the last byte is certainly always going to be 0, but it
> might not be reliably padded.

Right, my concern over comm is strictly about unterminated reads (i.e.
exposing memory contents stored after "comm" in the task_struct). I've not
been worried about "uninitialized content" exposure because the starting
contents have always been wiped and will (now) always end with a NUL,
so the worst exposure is seeing prior or racing bytes of whatever is
being written into comm concurrently.

> Why? It walks over the source twice. First at strlen() time, then at
> memcpy. So if the source isn't stable, the end result might have odd
> results with NUL characters in the middle.

Yeah, this just means it has greater potential to be garbled.

> And strscpy() really was *supposed* to be safe even in this case, and
> I thought it was until I looked closer.
> 
> But I think strscpy() can be saved.

Yeah, fixing the final NUL byte write is needed.

> Something (UNTESTED!) like the attached I think does the right thing.
> I added a couple of "READ_ONCE()" things to make it really super-clear
> that strscpy() reads the source exactly once, and to not allow any
> compiler re-materialization of the reads (although I think that when I
> asked people, it turns out neither gcc nor clang rematerialize memory
> accesses, so that READ_ONCE is likely more a documentation ad
> theoretical thing than a real thing).

This is fine, but it doesn't solve either an unstable source nor
concurrent writers to dest. If source changes out from under strscpy,
we can still copy a "torn" write. If destination changes out from under
strscpy, we just get a potentially interleaved output (but with the
NUL-write change, we never have a dest that _lacks_ a NUL terminator).

So yeah, let's change the loop as you have it. I'm fine with the
READ_ONCE() additions, but I'm not clear on what benefit it has.

> Hmm? I don't think your version is wrong, but I also think we'd be
> better off making our 'strscpy()' infrastructure explicitly safe wrt
> unstable source strings.

Agreed. I'll get this tested against our string handling selftests...

-- 
Kees Cook

Re: [PATCH] exec: Make sure task->comm is always NUL-terminated

[Linus Torvalds]

On Sat, 30 Nov 2024 at 13:05, Kees Cook <kees@kernel.org> wrote:
>
> Yeah, this just means it has greater potential to be garbled.

Garbled is fine. Id' just rather it be "consistently padded".

> This is fine, but it doesn't solve either an unstable source nor
> concurrent writers to dest.

Yeah, I guess concurrent writers will also cause possibly inconsistent padding.

Maybe we just don't care. As long as it's NUL-terminated, it's a
string. If somebody is messing with the kernel, they get to the
garbled string parts.

           Linus

RE: [PATCH] exec: Make sure task->comm is always NUL-terminated

[David Laight]

From: Kees Cook
> Sent: 30 November 2024 04:49
>
> Instead of adding a new use of the ambiguous strncpy(), we'd want to
> use memtostr_pad() which enforces being able to check at compile time
> that sizes are sensible, but this requires being able to see string
> buffer lengths. Instead of trying to inline __set_task_comm() (which
> needs to call trace and perf functions), just open-code it. But to
> make sure we're always safe, add compile-time checking like we already
> do for get_task_comm().
...
> Here's what I'd prefer to use to clean up set_task_comm(). I merged
> Linus and Eric's suggestions and open-coded memtostr_pad().
> ---
>  fs/exec.c             | 12 ++++++------
>  include/linux/sched.h |  9 ++++-----
>  io_uring/io-wq.c      |  2 +-
>  io_uring/sqpoll.c     |  2 +-
>  kernel/kthread.c      |  3 ++-
>  5 files changed, 14 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index e0435b31a811..5f16500ac325 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1200,16 +1200,16 @@ char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
>  EXPORT_SYMBOL_GPL(__get_task_comm);
> 
>  /*
> - * These functions flushes out all traces of the currently running executable
> - * so that a new one can be started
> + * This is unlocked -- the string will always be NUL-terminated, but
> + * may show overlapping contents if racing concurrent reads.
>   */
> -
>  void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
>  {
> -	task_lock(tsk);
> +	size_t len = min(strlen(buf), sizeof(tsk->comm) - 1);
> +
>  	trace_task_rename(tsk, buf);
> -	strscpy_pad(tsk->comm, buf, sizeof(tsk->comm));
> -	task_unlock(tsk);
> +	memcpy(tsk->comm, buf, len);
> +	memset(&tsk->comm[len], 0, sizeof(tsk->comm) - len);
>  	perf_event_comm(tsk, exec);

Why not do strscpy_pad() into a local char[16] and then do a 16 byte
memcpy() into the target buffer?

Then non-constant input data will always give a valid '\0' terminated string
regardless of how strscpy_pad() is implemented.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH] exec: Make sure task->comm is always NUL-terminated

[Linus Torvalds]

On Fri, 29 Nov 2024 at 23:15, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> And yes, we could make the word-at-a-time case also know about masking
> the last word, but it's kind of annoying and depends on byte ordering.

Actually, it turned out to be really trivial to do. It does depend on
byte order, but not in a very complex way.

Also, doing the memory accesses with READ_ONCE() might be good for
clarity, but it makes gcc have conniptions and makes the code
generation noticeably worse.

I'm not sure why, but gcc stops doing address generation in the memory
instruction for volatile accesses. I've seen that before, but
completely forgot about how odd the code generation becomes.

This actually generates quite good code - apart from the later
'memset()' by strscpy_pad().  Kind of sad, since the word-at-a-time
code by 'strscpy()' actually handles comm[] really well (the buffer is
a nice multiple of the word length), and extending it to padding would
be trivial.

The whole sized_strscpy_pad() macro is in fact all kinds of stupid. It does

        __wrote = sized_strscpy(__dst, __src, __count);
        if (__wrote >= 0 && __wrote < __count)

and that '__wrote' name is actively misleading, and the "__wrote <
__count" test is pointless.

The underlying sized_strscpy() function doesn't return how many
characters it wrote, it returns the length of the resulting string (or
error if it truncated it), so the return value is *always* smaller
than __count.

That's the whole point of the function, after all.

Oh well. I'll just commit my strscpy() improvement as a fix.

And I'll think about how to do the "pad" version better too. Just because.

                Linus

Re: [PATCH] exec: Make sure task->comm is always NUL-terminated

[Jens Axboe]

On 11/29/24 9:49 PM, Kees Cook wrote:
> Using strscpy() meant that the final character in task->comm may be
> non-NUL for a moment before the "string too long" truncation happens.
> 
> Instead of adding a new use of the ambiguous strncpy(), we'd want to
> use memtostr_pad() which enforces being able to check at compile time
> that sizes are sensible, but this requires being able to see string
> buffer lengths. Instead of trying to inline __set_task_comm() (which
> needs to call trace and perf functions), just open-code it. But to
> make sure we're always safe, add compile-time checking like we already
> do for get_task_comm().

In terms of the io_uring changes, both of those looks fine to me. Feel
free to bundle it with something else. If you're still changing things,
then I do prefer = { }; rather than no space...

-- 
Jens Axboe