6.6-stable review patch. If anyone has any objections, please let me know.
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From: Mathieu Desnoyers mathieu.desnoyers@efficios.com
[ Upstream commit dec890089bf79a4954b61482715ee2d084364856 ]
The following race can cause rb_time_read() to observe a corrupted time stamp:
rb_time_cmpxchg() [...] if (!rb_time_read_cmpxchg(&t->msb, msb, msb2)) return false; if (!rb_time_read_cmpxchg(&t->top, top, top2)) return false; <interrupted before updating bottom> __rb_time_read() [...] do { c = local_read(&t->cnt); top = local_read(&t->top); bottom = local_read(&t->bottom); msb = local_read(&t->msb); } while (c != local_read(&t->cnt));
*cnt = rb_time_cnt(top);
/* If top and msb counts don't match, this interrupted a write */ if (*cnt != rb_time_cnt(msb)) return false; ^ this check fails to catch that "bottom" is still not updated.
So the old "bottom" value is returned, which is wrong.
Fix this by checking that all three of msb, top, and bottom 2-bit cnt values match.
The reason to favor checking all three fields over requiring a specific update order for both rb_time_set() and rb_time_cmpxchg() is because checking all three fields is more robust to handle partial failures of rb_time_cmpxchg() when interrupted by nested rb_time_set().
Link: https://lore.kernel.org/lkml/20231211201324.652870-1-mathieu.desnoyers@effic... Link: https://lore.kernel.org/linux-trace-kernel/20231212193049.680122-1-mathieu.d...
Fixes: f458a1453424e ("ring-buffer: Test last update in 32bit version of __rb_time_read()") Signed-off-by: Mathieu Desnoyers mathieu.desnoyers@efficios.com Signed-off-by: Steven Rostedt (Google) rostedt@goodmis.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/trace/ring_buffer.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index af08a1a411e3d..070566baa0ca4 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -644,8 +644,8 @@ static inline bool __rb_time_read(rb_time_t *t, u64 *ret, unsigned long *cnt)
*cnt = rb_time_cnt(top);
- /* If top and msb counts don't match, this interrupted a write */ - if (*cnt != rb_time_cnt(msb)) + /* If top, msb or bottom counts don't match, this interrupted a write */ + if (*cnt != rb_time_cnt(msb) || *cnt != rb_time_cnt(bottom)) return false;
/* The shift to msb will lose its cnt bits */