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Subject[PATCH v2] relay: handle alloc_percpu returning NULL in relay_open
alloc_percpu() may return NULL, which means chan->buf may be set to
NULL. In that case, when we do *per_cpu_ptr(chan->buf, ...), we
dereference an invalid pointer:

BUG: Unable to handle kernel data access at 0x7dae0000
Faulting instruction address: 0xc0000000003f3fec
NIP [c0000000003f3fec] relay_open+0x29c/0x600
LR [c0000000003f3fc0] relay_open+0x270/0x600
Call Trace:
[c000000054353a70] [c0000000003f3fb4] relay_open+0x264/0x600 (unreliable)
[c000000054353b00] [c000000000451764] __blk_trace_setup+0x254/0x600
[c000000054353bb0] [c000000000451b78] blk_trace_setup+0x68/0xa0
[c000000054353c10] [c0000000010da77c] sg_ioctl+0x7bc/0x2e80
[c000000054353cd0] [c000000000758cbc] do_vfs_ioctl+0x13c/0x1300
[c000000054353d90] [c000000000759f14] ksys_ioctl+0x94/0x130
[c000000054353de0] [c000000000759ff8] sys_ioctl+0x48/0xb0
[c000000054353e20] [c00000000000bcd0] system_call+0x5c/0x68

Check if alloc_percpu returns NULL.

This was found by syzkaller both on x86 and powerpc, and the reproducer
it found on powerpc is capable of hitting the issue as an unprivileged

Fixes: 017c59c042d0 ("relay: Use per CPU constructs for the relay channel buffer pointers")
Cc: Akash Goel <>
Cc: Andrew Donnellan <> # syzkaller-ppc64
Reviewed-by: Michael Ellerman <>
Reviewed-by: Andrew Donnellan <>
Cc: # v4.10+
Signed-off-by: Daniel Axtens <>


v2: drop the NOWARN.

There's a syz reproducer on the powerpc syzbot that eventually hits
the bug, but it can take up to an hour or so before it keels over on a
kernel with all the syzkaller debugging on, and even longer on a
production kernel. I have been able to reproduce it once on a stock
Ubuntu 5.0 ppc64le kernel.

CVE-2019-19462 has been assigned. While only the process doing the syscall
gets killed, it gets killed while holding the relay_channels_mutex,
so it blocks all future relay activity.
kernel/relay.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/kernel/relay.c b/kernel/relay.c
index ade14fb7ce2e..4b760ec16342 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -581,6 +581,11 @@ struct rchan *relay_open(const char *base_filename,
return NULL;

chan->buf = alloc_percpu(struct rchan_buf *);
+ if (!chan->buf) {
+ kfree(chan);
+ return NULL;
+ }
chan->n_subbufs = n_subbufs;
chan->subbuf_size = subbuf_size;
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