[PATCH 6.18 439/752] erofs: fix interlaced plain identification for encoded extents

Sasha Levin
2026-02-28T17:54:16Z

From: Gao Xiang <hsiangkao@linux.alibaba.com>

[ Upstream commit 4a2d046e4b13202a6301a993961f5b30ae4d7119 ]

Only plain data whose start position and on-disk physical length are
both aligned to the block size should be classified as interlaced
plain extents. Otherwise, it must be treated as shifted plain extents.

This issue was found by syzbot using a crafted compressed image
containing plain extents with unaligned physical lengths, which can
cause OOB read in z_erofs_transform_plain().

Reported-and-tested-by: syzbot+d988dc155e740d76a331@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/699d5714.050a0220.cdd3c.03e7.GAE@google.com
Fixes: 1d191b4ca51d ("erofs: implement encoded extent metadata")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/erofs/zmap.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
index c8d8e129eb4ba..30775502b56da 100644
--- a/fs/erofs/zmap.c
+++ b/fs/erofs/zmap.c
@@ -513,6 +513,7 @@ static int z_erofs_map_blocks_ext(struct inode *inode,
unsigned int recsz = z_erofs_extent_recsize(vi->z_advise);
erofs_off_t pos = round_up(Z_EROFS_MAP_HEADER_END(erofs_iloc(inode) +
vi->inode_isize + vi->xattr_isize), recsz);
+ unsigned int bmask = sb->s_blocksize - 1;
bool in_mbox = erofs_inode_in_metabox(inode);
erofs_off_t lend = inode->i_size;
erofs_off_t l, r, mid, pa, la, lstart;
@@ -596,17 +597,17 @@ static int z_erofs_map_blocks_ext(struct inode *inode,
map->m_flags |= EROFS_MAP_MAPPED |
EROFS_MAP_FULL_MAPPED | EROFS_MAP_ENCODED;
fmt = map->m_plen >> Z_EROFS_EXTENT_PLEN_FMT_BIT;
+ if (map->m_plen & Z_EROFS_EXTENT_PLEN_PARTIAL)
+ map->m_flags |= EROFS_MAP_PARTIAL_REF;
+ map->m_plen &= Z_EROFS_EXTENT_PLEN_MASK;
if (fmt)
map->m_algorithmformat = fmt - 1;
- else if (interlaced && !erofs_blkoff(sb, map->m_pa))
+ else if (interlaced && !((map->m_pa | map->m_plen) & bmask))
map->m_algorithmformat =
Z_EROFS_COMPRESSION_INTERLACED;
else
map->m_algorithmformat =
Z_EROFS_COMPRESSION_SHIFTED;
- if (map->m_plen & Z_EROFS_EXTENT_PLEN_PARTIAL)
- map->m_flags |= EROFS_MAP_PARTIAL_REF;
- map->m_plen &= Z_EROFS_EXTENT_PLEN_MASK;
}
}
map->m_llen = lend - map->m_la;
--
2.51.0