linux-kernel - [PATCH 5.9 206/252] xfs: fix forkoff miscalculation related to XFS

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Date:   Mon, 23 Nov 2020 13:22:36 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Gao Xiang <hsiangkao@...hat.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dennis Gilmore <dgilmore@...hat.com>
Subject: [PATCH 5.9 206/252] xfs: fix forkoff miscalculation related to XFS_LITINO(mp)

From: Gao Xiang <hsiangkao@...hat.com>

commit ada49d64fb3538144192181db05de17e2ffc3551 upstream.

Currently, commit e9e2eae89ddb dropped a (int) decoration from
XFS_LITINO(mp), and since sizeof() expression is also involved,
the result of XFS_LITINO(mp) is simply as the size_t type
(commonly unsigned long).

Considering the expression in xfs_attr_shortform_bytesfit():
  offset = (XFS_LITINO(mp) - bytes) >> 3;
let "bytes" be (int)340, and
    "XFS_LITINO(mp)" be (unsigned long)336.

on 64-bit platform, the expression is
  offset = ((unsigned long)336 - (int)340) >> 3 =
           (int)(0xfffffffffffffffcUL >> 3) = -1

but on 32-bit platform, the expression is
  offset = ((unsigned long)336 - (int)340) >> 3 =
           (int)(0xfffffffcUL >> 3) = 0x1fffffff
instead.

so offset becomes a large positive number on 32-bit platform, and
cause xfs_attr_shortform_bytesfit() returns maxforkoff rather than 0.

Therefore, one result is
  "ASSERT(new_size <= XFS_IFORK_SIZE(ip, whichfork));"

assertion failure in xfs_idata_realloc(), which was also the root
cause of the original bugreport from Dennis, see:
   https://bugzilla.redhat.com/show_bug.cgi?id=1894177

And it can also be manually triggered with the following commands:
  $ touch a;
  $ setfattr -n user.0 -v "`seq 0 80`" a;
  $ setfattr -n user.1 -v "`seq 0 80`" a

on 32-bit platform.

Fix the case in xfs_attr_shortform_bytesfit() by bailing out
"XFS_LITINO(mp) < bytes" in advance suggested by Eric and a misleading
comment together with this bugfix suggested by Darrick. It seems the
other users of XFS_LITINO(mp) are not impacted.

Fixes: e9e2eae89ddb ("xfs: only check the superblock version for dinode size calculation")
Cc: <stable@...r.kernel.org> # 5.7+
Reported-and-tested-by: Dennis Gilmore <dgilmore@...hat.com>
Reviewed-by: Christoph Hellwig <hch@....de>
Signed-off-by: Gao Xiang <hsiangkao@...hat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@...cle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/xfs/libxfs/xfs_attr_leaf.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/fs/xfs/libxfs/xfs_attr_leaf.c
+++ b/fs/xfs/libxfs/xfs_attr_leaf.c
@@ -515,7 +515,7 @@ xfs_attr_copy_value(
  *========================================================================*/

 /*
- * Query whether the requested number of additional bytes of extended
+ * Query whether the total requested number of attr fork bytes of extended
  * attribute space will be able to fit inline.
  *
  * Returns zero if not, else the di_forkoff fork offset to be used in the
@@ -535,6 +535,12 @@ xfs_attr_shortform_bytesfit(
 	int			maxforkoff;
 	int			offset;

+	/*
+	 * Check if the new size could fit at all first:
+	 */
+	if (bytes > XFS_LITINO(mp))
+		return 0;
+
 	/* rounded down */
 	offset = (XFS_LITINO(mp) - bytes) >> 3;