[v2] cifs: clear PF_MEMALLOC before exiting demultiplex thread

From: Eric Biggers <ebiggers@google.com>

Leaving PF_MEMALLOC set when exiting a kthread causes it to remain set
during do_exit().  That can confuse things.  For example, if BSD process
accounting is enabled, then it's possible for do_exit() to end up
calling ext4_write_inode().  That triggers the
WARN_ON_ONCE(current->flags & PF_MEMALLOC) there, as it assumes
(appropriately) that inodes aren't written when allocating memory.

This case was reported by syzbot at
https://lkml.kernel.org/r/0000000000000e7156059f751d7b@google.com.

Fix this in cifs_demultiplex_thread() by using the helper functions to
save and restore PF_MEMALLOC.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---

v2: added missing include of <linux/sched/mm.h>
    (I missed that I didn't actually have CONFIG_CIFS set...)

 fs/cifs/connect.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

merged into cifs-2.6.git for-next

running buildbot cifs/smb3 automated regression tests now

On Sun, Mar 8, 2020 at 12:17 AM Eric Biggers <ebiggers@kernel.org> wrote:
>
> From: Eric Biggers <ebiggers@google.com>
>
> Leaving PF_MEMALLOC set when exiting a kthread causes it to remain set
> during do_exit().  That can confuse things.  For example, if BSD process
> accounting is enabled, then it's possible for do_exit() to end up
> calling ext4_write_inode().  That triggers the
> WARN_ON_ONCE(current->flags & PF_MEMALLOC) there, as it assumes
> (appropriately) that inodes aren't written when allocating memory.
>
> This case was reported by syzbot at
> https://lkml.kernel.org/r/0000000000000e7156059f751d7b@google.com.
>
> Fix this in cifs_demultiplex_thread() by using the helper functions to
> save and restore PF_MEMALLOC.
>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>
> v2: added missing include of <linux/sched/mm.h>
>     (I missed that I didn't actually have CONFIG_CIFS set...)
>
>  fs/cifs/connect.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
> index 4804d1df8c1c..97b8eb585cf9 100644
> --- a/fs/cifs/connect.c
> +++ b/fs/cifs/connect.c
> @@ -21,6 +21,7 @@
>  #include <linux/fs.h>
>  #include <linux/net.h>
>  #include <linux/string.h>
> +#include <linux/sched/mm.h>
>  #include <linux/sched/signal.h>
>  #include <linux/list.h>
>  #include <linux/wait.h>
> @@ -1164,8 +1165,9 @@ cifs_demultiplex_thread(void *p)
>         struct task_struct *task_to_wake = NULL;
>         struct mid_q_entry *mids[MAX_COMPOUND];
>         char *bufs[MAX_COMPOUND];
> +       unsigned int noreclaim_flag;
>
> -       current->flags |= PF_MEMALLOC;
> +       noreclaim_flag = memalloc_noreclaim_save();
>         cifs_dbg(FYI, "Demultiplex PID: %d\n", task_pid_nr(current));
>
>         length = atomic_inc_return(&tcpSesAllocCount);
> @@ -1320,6 +1322,7 @@ cifs_demultiplex_thread(void *p)
>                 set_current_state(TASK_RUNNING);
>         }
>
> +       memalloc_noreclaim_restore(noreclaim_flag);
>         module_put_and_exit(0);
>  }
>
> --
> 2.25.1
>

On Sun, Mar 08, 2020 at 01:43:56PM -0500, Steve French wrote:
> merged into cifs-2.6.git for-next
> 
> running buildbot cifs/smb3 automated regression tests now
> 

Thanks.  FYI, I also sent a similar patch for an XFS kernel thread, and the XFS
folks requested more details in the commit message [1].  So I ended up trying to
reproduce this warning on both XFS and CIFS, and I added more details to both
commit messages.  So if it's not too late, please update the CIFS commit to the
v3 I'm sending out.  Thanks!

[1] https://lkml.kernel.org/linux-xfs/20200308230307.GM10776@dread.disaster.area

- Eric