From: Namjae Jeon linkinjeon@kernel.org
[ Upstream commit 79c1587b6cda74deb0c86fc7ba194b92958c793c ]
syzbot created an exfat image with cluster bits not set for the allocation bitmap. exfat-fs reads and uses the allocation bitmap without checking this. The problem is that if the start cluster of the allocation bitmap is 6, cluster 6 can be allocated when creating a directory with mkdir. exfat zeros out this cluster in exfat_mkdir, which can delete existing entries. This can reallocate the allocated entries. In addition, the allocation bitmap is also zeroed out, so cluster 6 can be reallocated. This patch adds exfat_test_bitmap_range to validate that clusters used for the allocation bitmap are correctly marked as in-use.
Reported-by: syzbot+a725ab460fc1def9896f@syzkaller.appspotmail.com Tested-by: syzbot+a725ab460fc1def9896f@syzkaller.appspotmail.com Reviewed-by: Yuezhang Mo Yuezhang.Mo@sony.com Reviewed-by: Sungjong Seo sj1557.seo@samsung.com Signed-off-by: Namjae Jeon linkinjeon@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org ---
LLM Generated explanations, may be completely bogus:
YES - The commit adds `exfat_test_bitmap_range()` to verify that every cluster backing the allocation bitmap file has its allocation bit set before the filesystem accepts the bitmap (`fs/exfat/balloc.c:29`). Without this guard, the mount path would happily proceed even when the bitmap’s own clusters are marked free, exactly the corruption pattern syzbot reported. - The new helper simply walks the existing bitmap pages (`sbi->vol_amap`) and checks the relevant bits with the existing macros; on any mismatch it rejects the volume with `-EIO`, preventing us from ever reaching the allocator that can hand the bitmap’s cluster to new directories (`fs/exfat/balloc.c:108`, `fs/exfat/balloc.c:114`). This is a small, self-contained mount-time validation step. - The bug being fixed is high severity: when the bitmap cluster is falsely free, `exfat_alloc_cluster()` can select it and zero the data while creating a directory (`fs/exfat/fatent.c:381` onward), destroying the bitmap and any directory entries stored there. The patch blocks that corruption before it can happen. - Risk of regression is minimal—the helper only reads data we already loaded, relies on longstanding helpers/macros, and touches no runtime paths once the bitmap validates. If the check fails we already have to bail out because the on-disk image is inconsistent; no new behavior appears for well-formed volumes. - The change stands on its own (no functional dependencies on later commits), fixes a real user-visible corruption scenario, and adheres to stable-tree guidance (bug fix, limited scope, no architectural churn). Backporting will materially improve resilience of exFAT mounts against malformed media.
fs/exfat/balloc.c | 72 +++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 60 insertions(+), 12 deletions(-)
diff --git a/fs/exfat/balloc.c b/fs/exfat/balloc.c index cc01556c9d9b3..071448adbd5d9 100644 --- a/fs/exfat/balloc.c +++ b/fs/exfat/balloc.c @@ -26,12 +26,55 @@ /* * Allocation Bitmap Management Functions */ +static bool exfat_test_bitmap_range(struct super_block *sb, unsigned int clu, + unsigned int count) +{ + struct exfat_sb_info *sbi = EXFAT_SB(sb); + unsigned int start = clu; + unsigned int end = clu + count; + unsigned int ent_idx, i, b; + unsigned int bit_offset, bits_to_check; + __le_long *bitmap_le; + unsigned long mask, word; + + if (!is_valid_cluster(sbi, start) || !is_valid_cluster(sbi, end - 1)) + return false; + + while (start < end) { + ent_idx = CLUSTER_TO_BITMAP_ENT(start); + i = BITMAP_OFFSET_SECTOR_INDEX(sb, ent_idx); + b = BITMAP_OFFSET_BIT_IN_SECTOR(sb, ent_idx); + + bitmap_le = (__le_long *)sbi->vol_amap[i]->b_data; + + /* Calculate how many bits we can check in the current word */ + bit_offset = b % BITS_PER_LONG; + bits_to_check = min(end - start, + (unsigned int)(BITS_PER_LONG - bit_offset)); + + /* Create a bitmask for the range of bits to check */ + if (bits_to_check >= BITS_PER_LONG) + mask = ~0UL; + else + mask = ((1UL << bits_to_check) - 1) << bit_offset; + word = lel_to_cpu(bitmap_le[b / BITS_PER_LONG]); + + /* Check if all bits in the mask are set */ + if ((word & mask) != mask) + return false; + + start += bits_to_check; + } + + return true; +} + static int exfat_allocate_bitmap(struct super_block *sb, struct exfat_dentry *ep) { struct exfat_sb_info *sbi = EXFAT_SB(sb); long long map_size; - unsigned int i, need_map_size; + unsigned int i, j, need_map_size; sector_t sector;
sbi->map_clu = le32_to_cpu(ep->dentry.bitmap.start_clu); @@ -58,20 +101,25 @@ static int exfat_allocate_bitmap(struct super_block *sb, sector = exfat_cluster_to_sector(sbi, sbi->map_clu); for (i = 0; i < sbi->map_sectors; i++) { sbi->vol_amap[i] = sb_bread(sb, sector + i); - if (!sbi->vol_amap[i]) { - /* release all buffers and free vol_amap */ - int j = 0; - - while (j < i) - brelse(sbi->vol_amap[j++]); - - kvfree(sbi->vol_amap); - sbi->vol_amap = NULL; - return -EIO; - } + if (!sbi->vol_amap[i]) + goto err_out; }
+ if (exfat_test_bitmap_range(sb, sbi->map_clu, + EXFAT_B_TO_CLU_ROUND_UP(map_size, sbi)) == false) + goto err_out; + return 0; + +err_out: + j = 0; + /* release all buffers and free vol_amap */ + while (j < i) + brelse(sbi->vol_amap[j++]); + + kvfree(sbi->vol_amap); + sbi->vol_amap = NULL; + return -EIO; }
int exfat_load_bitmap(struct super_block *sb)