From 11816f7131c876b911605a8dc8b0a8835ed0d715 Mon Sep 17 00:00:00 2001 From: DaeMyung Kang Date: Wed, 6 May 2026 18:24:48 +0900 Subject: [PATCH] ntfs: fix out-of-bounds write in ntfs_rl_collapse_range() merge path ntfs_rl_collapse_range() merges the run on the left of the collapsed region with the run on its right when they are contiguous. The contiguous check chooses a clamped index when @new_1st_cnt is 0: i = new_1st_cnt == 0 ? 1 : new_1st_cnt; if (ntfs_rle_lcn_contiguous(&new_rl[i - 1], &new_rl[i])) { but the merge itself uses the unclamped value: s_rl = &new_rl[new_1st_cnt - 1]; s_rl->length += s_rl[1].length; When @new_1st_cnt is 0 this computes &new_rl[-1] and writes 8 bytes before the kvcalloc() runlist buffer. The path is reachable through fallocate(FALLOC_FL_COLLAPSE_RANGE) starting at vcn 0 against an attribute whose first run after the collapsed region and the following run are holes. In that case ntfs_rle_lcn_contiguous() returns true because both checked entries are LCN_HOLE, so the merge path is entered with @new_1st_cnt still 0. Such consecutive holes do not occur on a well-formed runlist (NTFS keeps runlists coalesced in memory), so this OOB path is only reachable from a crafted volume. A normal runlist has no element to the left of vcn 0, so the left/right merge is not valid when @new_1st_cnt is 0. Require @new_1st_cnt to be positive before checking or performing the merge. This skips the merge entirely in that case instead of clamping the merge target. The out-of-bounds write can corrupt an adjacent slab object. On a non-KASAN kernel, it is reachable after a crafted NTFS volume has been mounted read-write with the legacy fs/ntfs driver, by a local user that has write access to the crafted file. Fixes: 11ccc9107dc4 ("ntfs: update runlist handling and cluster allocator") Suggested-by: Hyunchul Lee Signed-off-by: DaeMyung Kang Reviewed-by: Hyunchul Lee Signed-off-by: Namjae Jeon --- fs/ntfs/runlist.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/fs/ntfs/runlist.c b/fs/ntfs/runlist.c index da21dbeaaf66..e7de3d01257e 100644 --- a/fs/ntfs/runlist.c +++ b/fs/ntfs/runlist.c @@ -2056,10 +2056,11 @@ struct runlist_element *ntfs_rl_collapse_range(struct runlist_element *dst_rl, i * consists of holes. */ merge_cnt = 0; - i = new_1st_cnt == 0 ? 1 : new_1st_cnt; - if (ntfs_rle_lcn_contiguous(&new_rl[i - 1], &new_rl[i])) { - /* Merge right and left */ - s_rl = &new_rl[new_1st_cnt - 1]; + if (new_1st_cnt > 0 && + ntfs_rle_lcn_contiguous(&new_rl[new_1st_cnt - 1], + &new_rl[new_1st_cnt])) { + /* Merge right and left. */ + s_rl = &new_rl[new_1st_cnt - 1]; s_rl->length += s_rl[1].length; merge_cnt = 1; }