[RFC PATCH 05/10] perf/core: Don't unwind the user stack without an mm

[James Houghton <jthoughton@google.com>]

get_perf_callchain() only checks is_user_task(current) when the regs it
is given aren't user mode regs. However, BPF's bpf_get_task_stack() (and
bpf_get_task_stack_sleepable()) pass task_pt_regs(current) when called on
the current task, which are user mode regs for a user task, even if that
task has already gone through exit_mm(), e.g. when called from a program
attached to exit_files() or to the dying task's final sched_switch.

perf_callchain_user() then unwinds the user stack without an mm. That
access does not reliably fail: the user half of the address space
belongs to whatever mm happens to be lazily loaded, if any, so the
unwinder may read another task's memory. This affects e.g. arm64,
riscv, loongarch and csky, whose perf_callchain_user() doesn't check
current->mm itself. perf events themselves are not affected, as
perf_callchain() already clears the user callchain request when
current is not a user task with an mm.

Check is_user_task(current) regardless of the regs' mode.

Fixes: fa28dcb82a38 ("bpf: Introduce helper bpf_get_task_stack()")
Signed-off-by: James Houghton <jthoughton@google.com>
---
 kernel/events/callchain.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 9d24b6e0c91f..a451ca2a5573 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -245,11 +245,15 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 	}
 
 	if (user && !crosstask) {
-		if (!user_mode(regs)) {
-			if (!is_user_task(current))
-				goto exit_put;
+		/*
+		 * Even if @regs are user mode regs, e.g. task_pt_regs(current)
+		 * passed in by bpf_get_task_stack(), current may already have
+		 * gone through exit_mm().
+		 */
+		if (!is_user_task(current))
+			goto exit_put;
+		if (!user_mode(regs))
 			regs = task_pt_regs(current);
-		}
 
 		if (defer_cookie) {
 			/*
-- 
2.56.0.307.g5fe7bd680c-goog