[PATCH net-next] crypto: Fix af_alg_sendmsg(MSG_SPLICE_PAGES) sglist limit

[PATCH net-next] crypto: Fix af_alg_sendmsg(MSG_SPLICE_PAGES) sglist limit

[David Howells]

When af_alg_sendmsg() calls extract_iter_to_sg(), it passes MAX_SGL_ENTS as
the maximum number of elements that may be written to, but some of the
elements may already have been used (as recorded in sgl->cur), so
extract_iter_to_sg() may end up overrunning the scatterlist.

Fix this to limit the number of elements to "MAX_SGL_ENTS - sgl->cur".

Note: It probably makes sense in future to alter the behaviour of
extract_iter_to_sg() to stop if "sgtable->nents >= sg_max" instead, but
this is a smaller fix for now.

The bug causes errors looking something like:

BUG: KASAN: slab-out-of-bounds in sg_assign_page include/linux/scatterlist.h:109 [inline]
BUG: KASAN: slab-out-of-bounds in sg_set_page include/linux/scatterlist.h:139 [inline]
BUG: KASAN: slab-out-of-bounds in extract_bvec_to_sg lib/scatterlist.c:1183 [inline]
BUG: KASAN: slab-out-of-bounds in extract_iter_to_sg lib/scatterlist.c:1352 [inline]
BUG: KASAN: slab-out-of-bounds in extract_iter_to_sg+0x17a6/0x1960 lib/scatterlist.c:1339

Fixes: bf63e250c4b1 ("crypto: af_alg: Support MSG_SPLICE_PAGES")
Reported-by: syzbot+6efc50cc1f8d718d6cb7@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/000000000000b2585a05fdeb8379@google.com/
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: syzbot+6efc50cc1f8d718d6cb7@syzkaller.appspotmail.com
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-crypto@vger.kernel.org
cc: netdev@vger.kernel.org
---
 crypto/af_alg.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/crypto/af_alg.c b/crypto/af_alg.c
index 7d4b6016b83d..cdb1dcc5dd1a 100644
--- a/crypto/af_alg.c
+++ b/crypto/af_alg.c
@@ -1043,7 +1043,7 @@ int af_alg_sendmsg(struct socket *sock, struct msghdr *msg, size_t size,
 			};
 
 			plen = extract_iter_to_sg(&msg->msg_iter, len, &sgtable,
-						  MAX_SGL_ENTS, 0);
+						  MAX_SGL_ENTS - sgl->cur, 0);
 			if (plen < 0) {
 				err = plen;
 				goto unlock;

Re: [PATCH net-next] crypto: Fix af_alg_sendmsg(MSG_SPLICE_PAGES) sglist limit

[Herbert Xu]

On Thu, Jun 15, 2023 at 10:08:54PM +0100, David Howells wrote:
> When af_alg_sendmsg() calls extract_iter_to_sg(), it passes MAX_SGL_ENTS as
> the maximum number of elements that may be written to, but some of the
> elements may already have been used (as recorded in sgl->cur), so
> extract_iter_to_sg() may end up overrunning the scatterlist.
> 
> Fix this to limit the number of elements to "MAX_SGL_ENTS - sgl->cur".
> 
> Note: It probably makes sense in future to alter the behaviour of
> extract_iter_to_sg() to stop if "sgtable->nents >= sg_max" instead, but
> this is a smaller fix for now.
> 
> The bug causes errors looking something like:
> 
> BUG: KASAN: slab-out-of-bounds in sg_assign_page include/linux/scatterlist.h:109 [inline]
> BUG: KASAN: slab-out-of-bounds in sg_set_page include/linux/scatterlist.h:139 [inline]
> BUG: KASAN: slab-out-of-bounds in extract_bvec_to_sg lib/scatterlist.c:1183 [inline]
> BUG: KASAN: slab-out-of-bounds in extract_iter_to_sg lib/scatterlist.c:1352 [inline]
> BUG: KASAN: slab-out-of-bounds in extract_iter_to_sg+0x17a6/0x1960 lib/scatterlist.c:1339
> 
> Fixes: bf63e250c4b1 ("crypto: af_alg: Support MSG_SPLICE_PAGES")
> Reported-by: syzbot+6efc50cc1f8d718d6cb7@syzkaller.appspotmail.com
> Link: https://lore.kernel.org/r/000000000000b2585a05fdeb8379@google.com/
> Signed-off-by: David Howells <dhowells@redhat.com>
> Tested-by: syzbot+6efc50cc1f8d718d6cb7@syzkaller.appspotmail.com
> cc: Herbert Xu <herbert@gondor.apana.org.au>
> cc: "David S. Miller" <davem@davemloft.net>
> cc: Eric Dumazet <edumazet@google.com>
> cc: Jakub Kicinski <kuba@kernel.org>
> cc: Paolo Abeni <pabeni@redhat.com>
> cc: Jens Axboe <axboe@kernel.dk>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: linux-crypto@vger.kernel.org
> cc: netdev@vger.kernel.org
> ---
>  crypto/af_alg.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH net-next] crypto: Fix af_alg_sendmsg(MSG_SPLICE_PAGES) sglist limit

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by David S. Miller <davem@davemloft.net>:

On Thu, 15 Jun 2023 22:08:54 +0100 you wrote:
> When af_alg_sendmsg() calls extract_iter_to_sg(), it passes MAX_SGL_ENTS as
> the maximum number of elements that may be written to, but some of the
> elements may already have been used (as recorded in sgl->cur), so
> extract_iter_to_sg() may end up overrunning the scatterlist.
> 
> Fix this to limit the number of elements to "MAX_SGL_ENTS - sgl->cur".
> 
> [...]

Here is the summary with links:
  - [net-next] crypto: Fix af_alg_sendmsg(MSG_SPLICE_PAGES) sglist limit
    https://git.kernel.org/netdev/net-next/c/4380499218c6

You are awesome, thank you!
-- 
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