Overview
About vulnerability
In the Linux kernel, the following vulnerability has been resolved:
ocfs2: fix readdir position truncation on 32-bit kernels
In ocfs2_dir_foreach_blk_el(), the directory cookie position is rebuilt with
ctx->pos = (ctx->pos & ~(sb->s_blocksize - 1)) | offset;
ctx->pos is loff_t (signed 64-bit), while sb->s_blocksize is
unsigned long. On 32-bit kernels unsigned long is 32-bit, so the mask
~(sb->s_blocksize - 1)
is computed as a 32-bit unsigned value (e.g. 0xfffff000 for a 4 KiB
block size). In the AND expression with the 64-bit ctx->pos, that
unsigned operand is zero-extended to 64 bits per the usual arithmetic
conversions, yielding 0x00000000fffff000. The high 32 bits of
ctx->pos are silently cleared, even though directory size is
allowed to exceed 4 GiB.
When readdir() crosses the 4 GiB boundary on a 32-bit kernel the position is reset back into the first 4 GiB block, making the re-validation path re-enumerate already-returned dirents indefinitely.
This is ocfs2_dir_foreach_blk_el(), the extent-list readdir path taken for all non-inline directories, so a directory large enough to cross 4 GiB reaches it.
This is the same class of bug that commit 3dce5bb82c97 (“exfat: Fix bitwise operation having different size”) fixed in exfat, and the fix mirrors the equivalent ext4 fix in this series. Cast the operand to loff_t so the mask is 64-bit before the AND:
ctx->pos = (ctx->pos & ~((loff_t)sb->s_blocksize - 1)) | offset;
64-bit kernels are unaffected.
Details
- Affected product:
- Debian 10 ELS , Debian 11 ELS , Ubuntu 16.04 ELS , Ubuntu 18.04 ELS , Ubuntu 20.04 ELS
- Affected packages:
- linux @ 4.19.0 (+4 more)