On Thu, Jul 11, 2019 at 06:28:54PM +0300, Amir Goldstein wrote:

On Thu, Jul 11, 2019 at 5:00 PM Jan Kara jack@suse.cz wrote:

...

Hole puching currently evicts pages from page cache and then goes on to remove blocks from the inode. This happens under both XFS_IOLOCK_EXCL and XFS_MMAPLOCK_EXCL which provides appropriate serialization with racing reads or page faults. However there is currently nothing that prevents readahead triggered by fadvise() or madvise() from racing with the hole punch and instantiating page cache page after hole punching has evicted page cache in xfs_flush_unmap_range() but before it has removed blocks from the inode. This page cache page will be mapping soon to be freed block and that can lead to returning stale data to userspace or even filesystem corruption.

Fix the problem by protecting handling of readahead requests by XFS_IOLOCK_SHARED similarly as we protect reads.

CC: stable@vger.kernel.org Link: https://lore.kernel.org/linux-fsdevel/CAOQ4uxjQNmxqmtA_VbYW0Su9rKRk2zobJmahc... Reported-by: Amir Goldstein amir73il@gmail.com Signed-off-by: Jan Kara jack@suse.cz

---

Looks sane. (I'll let xfs developers offer reviewed-by tags)

...

fs/xfs/xfs_file.c | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c index 76748255f843..88fe3dbb3ba2 100644 --- a/fs/xfs/xfs_file.c +++ b/fs/xfs/xfs_file.c @@ -33,6 +33,7 @@ #include <linux/pagevec.h> #include <linux/backing-dev.h> #include <linux/mman.h> +#include <linux/fadvise.h>

static const struct vm_operations_struct xfs_file_vm_ops;

@@ -939,6 +940,24 @@ xfs_file_fallocate( return error; }

+STATIC int +xfs_file_fadvise(

•   struct file *file,
  

•   loff_t start,
  

•   loff_t end,
  

•   int advice)
  

Indentation needs fixing here.

...

+{

•   struct xfs_inode *ip = XFS_I(file_inode(file));
  

•   int ret;
  

•   /* Readahead needs protection from hole punching and similar ops */
  

•   if (advice == POSIX_FADV_WILLNEED)
  

•           xfs_ilock(ip, XFS_IOLOCK_SHARED);
  

It's good to fix this race, but at the same time I wonder what's the impact to processes writing to one part of a file waiting on IOLOCK_EXCL while readahead holds IOLOCK_SHARED?

(bluh bluh range locks ftw bluh bluh)

Do we need a lock for DONTNEED? I think the answer is that you have to lock the page to drop it and that will protect us from <myriad punch and truncate spaghetti> ... ?

...

•   ret = generic_fadvise(file, start, end, advice);
  

•   if (advice == POSIX_FADV_WILLNEED)
  

•           xfs_iunlock(ip, XFS_IOLOCK_SHARED);
  

Maybe it'd be better to do:

int lockflags = 0;

if (advice == POSIX_FADV_WILLNEED) { lockflags = XFS_IOLOCK_SHARED; xfs_ilock(ip, lockflags); }

ret = generic_fadvise(file, start, end, advice);

if (lockflags) xfs_iunlock(ip, lockflags);

Just in case we some day want more or different types of inode locks?

--D

...

•   return ret;
  

+}

STATIC loff_t xfs_file_remap_range( @@ -1235,6 +1254,7 @@ const struct file_operations xfs_file_operations = { .fsync = xfs_file_fsync, .get_unmapped_area = thp_get_unmapped_area, .fallocate = xfs_file_fallocate,

•   .fadvise        = xfs_file_fadvise,
    .remap_file_range = xfs_file_remap_range,
  

};

-- 2.16.4