[PATCH net] sunrpc: Use GFP_NOFS to prevent use of current->task_frag.

[Guillaume Nault <gnault@xxxxxxxxxx>]

Commit a1231fda7e94 ("SUNRPC: Set memalloc_nofs_save() on all
rpciod/xprtiod jobs") stopped setting sk->sk_allocation explicitly in
favor of using memalloc_nofs_save()/memalloc_nofs_restore() critical
sections.

However, ->sk_allocation isn't used just by the memory allocator.
In particular, sk_page_frag() uses it to figure out if it can return
the page_frag from current or if it has to use the socket one.
With ->sk_allocation set to the default GFP_KERNEL, sk_page_frag() now
returns current->page_frag, which might already be in use in the
current context if the call happens during memory reclaim.

Fix this by setting ->sk_allocation to GFP_NOFS.
Note that we can't just instruct sk_page_frag() to look at
current->flags, because it could generate a cache miss, thus slowing
down the TCP fast path.

This is similar to the problems fixed by the following two commits:
  * cifs: commit dacb5d8875cc ("tcp: fix page frag corruption on page
    fault").
  * nbd: commit 20eb4f29b602 ("net: fix sk_page_frag() recursion from
    memory reclaim").

Link: https://lore.kernel.org/netdev/b4d8cb09c913d3e34f853736f3f5628abfd7f4b6.1656699567.git.gnault@xxxxxxxxxx/
Fixes: a1231fda7e94 ("SUNRPC: Set memalloc_nofs_save() on all rpciod/xprtiod jobs")
Signed-off-by: Guillaume Nault <gnault@xxxxxxxxxx>
---
 net/sunrpc/xprtsock.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
index e976007f4fd0..1bd3048d43ae 100644
--- a/net/sunrpc/xprtsock.c
+++ b/net/sunrpc/xprtsock.c
@@ -1882,6 +1882,7 @@ static int xs_local_finish_connecting(struct rpc_xprt *xprt,
 		sk->sk_write_space = xs_udp_write_space;
 		sk->sk_state_change = xs_local_state_change;
 		sk->sk_error_report = xs_error_report;
+		sk->sk_allocation = GFP_NOFS;
 
 		xprt_clear_connected(xprt);
 
@@ -2083,6 +2084,7 @@ static void xs_udp_finish_connecting(struct rpc_xprt *xprt, struct socket *sock)
 		sk->sk_user_data = xprt;
 		sk->sk_data_ready = xs_data_ready;
 		sk->sk_write_space = xs_udp_write_space;
+		sk->sk_allocation = GFP_NOFS;
 
 		xprt_set_connected(xprt);
 
@@ -2250,6 +2252,7 @@ static int xs_tcp_finish_connecting(struct rpc_xprt *xprt, struct socket *sock)
 		sk->sk_state_change = xs_tcp_state_change;
 		sk->sk_write_space = xs_tcp_write_space;
 		sk->sk_error_report = xs_error_report;
+		sk->sk_allocation = GFP_NOFS;
 
 		/* socket options */
 		sock_reset_flag(sk, SOCK_LINGER);
-- 
2.21.3