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From: Daniel Axtens <>
	Daniel Axtens <>,,,,
Subject: [PATCH] relay: handle alloc_percpu returning NULL in relay_open
Date: Fri, 29 Nov 2019 12:37:45 +1100	[thread overview]
Message-ID: <> (raw)

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. Because we can readily catch and
handle this situation, switch to alloc_cpu_gfp and pass in __GFP_NOWARN.

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
Cc: # v4.10+
Signed-off-by: Daniel Axtens <>


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.

I will ask MITRE for a CVE - 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 | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/relay.c b/kernel/relay.c
index ade14fb7ce2e..a376cc6b54ec 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -580,7 +580,13 @@ struct rchan *relay_open(const char *base_filename,
 	if (!chan)
 		return NULL;
-	chan->buf = alloc_percpu(struct rchan_buf *);
+	chan->buf = alloc_percpu_gfp(struct rchan_buf *,
+				     GFP_KERNEL | __GFP_NOWARN);
+	if (!chan->buf) {
+		kfree(chan);
+		return NULL;
+	}
 	chan->version = RELAYFS_CHANNEL_VERSION;
 	chan->n_subbufs = n_subbufs;
 	chan->subbuf_size = subbuf_size;

             reply	other threads:[~2019-11-29  1:37 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-29  1:37 Daniel Axtens [this message]
2019-11-29  4:59 ` Michael Ellerman
2019-11-29 12:42   ` Andrew Donnellan
2019-11-30  6:04 ` Daniel Axtens
2019-12-23 16:36 ` Guenter Roeck
2019-12-24  0:26   ` Daniel Axtens

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