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Ext4 will always create ext4 extended attributes which do not have a
value (where e_value_size is zero) with e_value_offs set to zero. In
most places e_value_offs will not be used in a substantive way if
e_value_size is zero.
There was one exception to this, which is in ext4_xattr_set_entry(),
where if there is a maliciously crafted file system where there is an
extended attribute with e_value_offs is non-zero and e_value_size is
0, the attempt to remove this xattr will result in a negative value
getting passed to memmove, leading to the following sadness:
[ 41.225365] EXT4-fs (loop0): mounted filesystem with ordered data mode. Opts: (null)
[ 44.538641] BUG: unable to handle kernel paging request at ffff9ec9a3000000
[ 44.538733] IP: __memmove+0x81/0x1a0
[ 44.538755] PGD 1249bd067 P4D 1249bd067 PUD 1249c1067 PMD 80000001230000e1
[ 44.538793] Oops: 0003 [#1] SMP PTI
[ 44.539074] CPU: 0 PID: 1470 Comm: poc Not tainted 4.16.0-rc1+ #1
...
[ 44.539475] Call Trace:
[ 44.539832] ext4_xattr_set_entry+0x9e7/0xf80
...
[ 44.539972] ext4_xattr_block_set+0x212/0xea0
...
[ 44.540041] ext4_xattr_set_handle+0x514/0x610
[ 44.540065] ext4_xattr_set+0x7f/0x120
[ 44.540090] __vfs_removexattr+0x4d/0x60
[ 44.540112] vfs_removexattr+0x75/0xe0
[ 44.540132] removexattr+0x4d/0x80
...
[ 44.540279] path_removexattr+0x91/0xb0
[ 44.540300] SyS_removexattr+0xf/0x20
[ 44.540322] do_syscall_64+0x71/0x120
[ 44.540344] entry_SYSCALL_64_after_hwframe+0x21/0x86
https://bugzilla.kernel.org/show_bug.cgi?id=199347
This addresses CVE-2018-10840.
Reported-by: "Xu, Wen" <wen.xu@gatech.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Cc: stable@kernel.org
Fixes: dec214d00e ("ext4: xattr inode deduplication")
Linux kernel
============
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