[PATCH] perf: Fix refcount warning on event->mmap_count increment

[Will Rosenberg]

When calling refcount_inc(&event->mmap_count) inside perf_mmap_rb(), the
following warning is triggered:

	refcount_t: addition on 0; use-after-free.
	WARNING: lib/refcount.c:25

PoC:

    struct perf_event_attr attr = {0};
    int fd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0);
    mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    int victim = syscall(__NR_perf_event_open, &attr, 0, -1, fd,
                         PERF_FLAG_FD_OUTPUT);
    mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED, victim, 0);

This occurs when creating a group member event with the flag
PERF_FLAG_FD_OUTPUT. The group leader should be mmap-ed and then mmap-ing
the event triggers the warning.

Since the event has copied the output_event in perf_event_set_output(),
event->rb is set. As a result, perf_mmap_rb() calls
refcount_inc(&event->mmap_count) when event->mmap_count = 0.

Account for the case when event->mmap_count = 0. This patch goes against
the design philosophy of the refcount library by re-enabling an empty
refcount, but the patch remains inline with the current treatment of
mmap_count.

Signed-off-by: Will Rosenberg <whrosenb@xxxxxxx>
---

Notes:
    I also have a related concern about code that handles the mmap_count.
    In perf_mmap_close(), if refcount_dec_and_mutex_lock() decrements
    event->mmap_count to zero, then event->rb is set to NULL. This
    effectively undos our output_event copy. However, is this desired
    behavior? Should event->rb remain unchanged since it may still be
    mmap-ed by other events and can still be used?

 kernel/events/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 376fb07d869b..49709b627b1f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7279,7 +7279,8 @@ static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,
 			 * multiple times.
 			 */
 			perf_mmap_account(vma, user_extra, extra);
-			refcount_inc(&event->mmap_count);
+			if (!refcount_inc_not_zero(&event->mmap_count))
+				refcount_set(&event->mmap_count, 1);
 			return 0;
 		}
 

base-commit: 538254cd98afb31b09c4cc58219217d8127c79be
-- 
2.34.1