From: Filipe Manana fdmanana@suse.com
[ Upstream commit 41d69d4d78d8b179bf3bcdfc56d28a12b3a608d2 ]
If split_state() returned an error we call extent_io_tree_panic() which will trigger a BUG() call. However if CONFIG_BUG is disabled, which is an uncommon and exotic scenario, then we fallthrough and hit a use after free when calling set_state_bits() since the extent state record which the local variable 'prealloc' points to was freed by split_state().
So jump to the label 'out' after calling extent_io_tree_panic() and set the 'prealloc' pointer to NULL since split_state() has already freed it when it hit an error.
Signed-off-by: Filipe Manana fdmanana@suse.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Sasha Levin sashal@kernel.org ---
**YES** This commit should be backported to stable kernel trees. Here's my extensive analysis: ## Analysis of the Code Changes The commit fixes a critical use-after-free bug in the btrfs filesystem's extent I/O tree management. Examining the specific code changes: **Problem Location**: In `fs/btrfs/extent-io-tree.c`, function `set_extent_bit()` around lines 1254-1256: ```c ret = split_state(tree, state, prealloc, end + 1); if (ret) extent_io_tree_panic(tree, state, "split", ret); ``` **The Bug**: After `split_state()` fails and `extent_io_tree_panic()` is called, the code continues to execute `set_state_bits(tree, prealloc, bits, changeset)` on line 1258. However, when `split_state()` fails, it frees the `prealloc` extent state, making this a use-after-free vulnerability. **The Fix**: The commit adds proper error handling: ```c ret = split_state(tree, state, prealloc, end + 1); if (ret) { extent_io_tree_panic(tree, state, "split", ret); prealloc = NULL; goto out; } ``` ## Why This Should Be Backported ### 1. **Critical Security Issue** - **Use-after-free vulnerability**: This is a serious memory safety issue that can lead to kernel crashes, data corruption, or potentially exploitable conditions - **Affects btrfs filesystem**: A widely used filesystem in production environments ### 2. **Specific Edge Case Scenario** From my examination of the kernel code: - `extent_io_tree_panic()` calls `btrfs_panic()` which calls `BUG()` - When `CONFIG_BUG` is disabled (uncommon but possible), `BUG()` becomes a no-op loop instead of halting execution - This allows execution to continue to the use-after-free code ### 3. **Minimal Risk Change** - **Small, targeted fix**: Only adds 3 lines of code - **Clear logic**: Sets pointer to NULL and jumps to cleanup - **No functional changes**: Doesn't alter normal operation paths - **No architectural changes**: Pure bug fix without affecting subsystem design ### 4. **Comparison with Similar Commits** Looking at the reference commits: - Similar to "Similar Commit #2" (marked YES) which also improved error handling in extent bit operations - Unlike commits marked NO, this doesn't add features or make architectural changes - Follows the same pattern as other accepted backports for memory safety fixes ### 5. **Clear Bug Fix Criteria** - **Fixes a real bug**: Use-after-free is a concrete, exploitable issue - **Minimal scope**: Only affects error path in one function - **No side effects**: Change only affects already-failing code paths - **Well-understood impact**: Risk is contained to btrfs extent I/O operations ### 6. **Production Impact** - While `CONFIG_BUG` being disabled is rare, when it occurs this creates a serious vulnerability - btrfs is used in production environments where memory safety is critical - The fix prevents potential kernel crashes or memory corruption ### 7. **Stable Tree Compatibility** - **Self-contained**: Doesn't depend on other recent changes - **Backwards compatible**: No API changes - **Low regression risk**: Only affects error handling path that was already broken The commit perfectly fits stable tree criteria: it's a small, important bugfix with minimal risk that addresses a real security issue in a widely-used filesystem.
fs/btrfs/extent-io-tree.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/fs/btrfs/extent-io-tree.c b/fs/btrfs/extent-io-tree.c index bb3aaf610652a..5f9a43734812e 100644 --- a/fs/btrfs/extent-io-tree.c +++ b/fs/btrfs/extent-io-tree.c @@ -1252,8 +1252,11 @@ static int __set_extent_bit(struct extent_io_tree *tree, u64 start, u64 end, if (!prealloc) goto search_again; ret = split_state(tree, state, prealloc, end + 1); - if (ret) + if (ret) { extent_io_tree_panic(tree, state, "split", ret); + prealloc = NULL; + goto out; + }
set_state_bits(tree, prealloc, bits, changeset); cache_state(prealloc, cached_state);