Re: [PATCH] netfs: Fix early put of sink folio in netfs_read_gaps()

[Paulo Alcantara <pc@manguebit.org>]

David Howells <dhowells@redhat.com> writes:

> Fix netfs_read_gaps() to release the sink page it uses after waiting for
> the request to complete.  The way the sink page is used is that an
> ITER_BVEC-class iterator is created that has the gaps from the target folio
> at either end, but has the sink page tiled over the middle so that a single
> read op can fill in both gaps.
>
> The bug was found by KASAN detecting a UAF on the generic/075 xfstest in
> the cifsd kernel thread that handles reception of data from the TCP socket:
>
>  BUG: KASAN: use-after-free in _copy_to_iter+0x48a/0xa20
>  Write of size 885 at addr ffff888107f92000 by task cifsd/1285
>  CPU: 2 UID: 0 PID: 1285 Comm: cifsd Not tainted 7.0.0 #6 PREEMPT(lazy)
>  Call Trace:
>   dump_stack_lvl+0x5d/0x80
>   print_report+0x17f/0x4f1
>   kasan_report+0x100/0x1e0
>   kasan_check_range+0x10f/0x1e0
>   __asan_memcpy+0x3c/0x60
>   _copy_to_iter+0x48a/0xa20
>   __skb_datagram_iter+0x2c9/0x430
>   skb_copy_datagram_iter+0x6e/0x160
>   tcp_recvmsg_locked+0xce0/0x1130
>   tcp_recvmsg+0xeb/0x300
>   inet_recvmsg+0xcf/0x3a0
>   sock_recvmsg+0xea/0x100
>   cifs_readv_from_socket+0x3a6/0x4d0 [cifs]
>   cifs_read_iter_from_socket+0xdd/0x130 [cifs]
>   cifs_readv_receive+0xaad/0xb10 [cifs]
>   cifs_demultiplex_thread+0x1148/0x1740 [cifs]
>   kthread+0x1cf/0x210
>
> Fixes: ee4cdf7ba857 ("netfs: Speed up buffered reading")
> Reported-by: Steve French <sfrench@samba.org>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Paulo Alcantara <pc@manguebit.org>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: netfs@lists.linux.dev
> cc: linux-fsdevel@vger.kernel.org
> ---
>  fs/netfs/buffered_read.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)

Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>