From: KaFai Wan kafai.wan@linux.dev
[ Upstream commit d43ad9da8052eda714caa38f243adbf32a8614cb ]
When conditional jumps are performed on the same scalar register (e.g., r0 <= r0, r0 > r0, r0 < r0), the BPF verifier incorrectly attempts to adjust the register's min/max bounds. This leads to invalid range bounds and triggers a BUG warning.
The problematic BPF program: 0: call bpf_get_prandom_u32 1: w8 = 0x80000000 2: r0 &= r8 3: if r0 > r0 goto <exit>
The instruction 3 triggers kernel warning: 3: if r0 > r0 goto <exit> true_reg1: range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0) true_reg2: const tnum out of sync with range bounds u64=[0x0, 0xffffffffffffffff] s64=[0x8000000000000000, 0x7fffffffffffffff] var_off=(0x0, 0x0)
Comparing a register with itself should not change its bounds and for most comparison operations, comparing a register with itself has a known result (e.g., r0 == r0 is always true, r0 < r0 is always false).
Fix this by: 1. Enhance is_scalar_branch_taken() to properly handle branch direction computation for same register comparisons across all BPF jump operations 2. Adds early return in reg_set_min_max() to avoid bounds adjustment for unknown branch directions (e.g., BPF_JSET) on the same register
The fix ensures that unnecessary bounds adjustments are skipped, preventing the verifier bug while maintaining correct branch direction analysis.
Reported-by: Kaiyan Mei M202472210@hust.edu.cn Reported-by: Yinhao Hu dddddd@hust.edu.cn Closes: https://lore.kernel.org/all/1881f0f5.300df.199f2576a01.Coremail.kaiyanm@hust... Signed-off-by: KaFai Wan kafai.wan@linux.dev Acked-by: Eduard Zingerman eddyz87@gmail.com Link: https://lore.kernel.org/r/20251103063108.1111764-2-kafai.wan@linux.dev Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org ---
LLM Generated explanations, may be completely bogus:
Now I have a complete picture. Let me do a comprehensive summary and analysis:
## Complete Analysis
### 1. COMMIT MESSAGE ANALYSIS
**Subject:** "bpf: Skip bounds adjustment for conditional jumps on same scalar register"
**Key indicators:** - **Fix keyword:** The commit message clearly describes a bug fix ("incorrectly attempts", "invalid range bounds", "triggers a BUG warning") - **No stable tag:** The commit does not have explicit "Cc: stable@vger.kernel.org" - **No Fixes: tag:** No explicit Fixes: tag pointing to a prior commit - **Reported-by tags:** The commit has two "Reported-by:" tags and a "Closes:" link to a lore.kernel.org bug report - **Acked-by:** Eduard Zingerman (BPF maintainer) acknowledged this fix - **Merged by:** Alexei Starovoitov (BPF maintainer) signed off
The commit message describes: 1. When comparing a register with itself (e.g., `r0 > r0`), the verifier incorrectly adjusts bounds 2. This leads to invalid range bounds (umin > umax, etc.) 3. Triggers the `reg_bounds_sanity_check()` BUG warning
### 2. CODE CHANGE ANALYSIS
**Files changed:** 1 file (`kernel/bpf/verifier.c`)
**Two modifications:**
**Modification 1 - `is_scalar_branch_taken()` (lines 15996-16020 in diff):** Adds a new code block at the beginning of the function to handle same- register comparisons:
```c if (reg1 == reg2) { switch (opcode) { case BPF_JGE: case BPF_JLE: case BPF_JSGE: case BPF_JSLE: case BPF_JEQ: return 1; /* Always true: r0 >= r0, r0 <= r0, r0 == r0 */ case BPF_JGT: case BPF_JLT: case BPF_JSGT: case BPF_JSLT: case BPF_JNE: return 0; /* Always false: r0 > r0, r0 < r0, r0 != r0 */ case BPF_JSET: if (tnum_is_const(t1)) return t1.value != 0; else return (smin1 <= 0 && smax1 >= 0) ? -1 : 1; default: return -1; } } ```
This correctly determines branch direction for same-register comparisons: - `r0 == r0`, `r0 >= r0`, `r0 <= r0` are always true (return 1) - `r0 > r0`, `r0 < r0`, `r0 != r0` are always false (return 0) - `r0 JSET r0` depends on whether any bits are set
**Modification 2 - `reg_set_min_max()` (lines 16446-16452 in diff):** Adds early return when both register arguments point to the same memory:
```c /* We compute branch direction for same SCALAR_VALUE registers in - is_scalar_branch_taken(). For unknown branch directions (e.g., BPF_JSET) - on the same registers, we don't need to adjust the min/max values. */ if (false_reg1 == false_reg2) return 0; ```
This prevents `regs_refine_cond_op()` from corrupting bounds when called with the same pointer for both registers.
### 3. ROOT CAUSE OF THE BUG
When a BPF program compares a register with itself (e.g., `if r0 > r0`):
1. In `check_cond_jmp_op()`, both `dst_reg` and `src_reg` point to the same `bpf_reg_state` in memory because `®s[insn->dst_reg] == ®s[insn->src_reg]`
2. If `is_branch_taken()` returns -1 (unknown), `reg_set_min_max()` is called
3. `regs_refine_cond_op()` is then called with `reg1 == reg2` (same pointer)
4. For `BPF_JGT` (which becomes `BPF_JLT` after `flip_opcode`), the code does: ```c reg1->umax_value = min(reg1->umax_value, reg2->umax_value - 1); reg2->umin_value = max(reg1->umin_value + 1, reg2->umin_value); ```
Since `reg1 == reg2`, this becomes: - First line: `reg->umax_value = reg->umax_value - 1` (decreases max) - Second line reads the already-decreased `umax_value`, then: `reg->umin_value = max(reg->umin_value + 1, reg->umin_value)` (increases min)
5. This results in `umin_value > umax_value`, which is an invalid range!
6. `reg_bounds_sanity_check()` detects this and triggers a BUG warning
### 4. CLASSIFICATION
- **Type:** Bug fix - **Security impact:** Not a CVE, but triggers BUG (kernel warning/crash) - denial of service by unprivileged users (if unprivileged BPF is enabled) - **Exception categories:** None (this is a straightforward bug fix, not a device ID, quirk, DT update, or build fix)
### 5. SCOPE AND RISK ASSESSMENT
- **Lines changed:** ~30 new lines of code - **Files touched:** 1 file (`kernel/bpf/verifier.c`) - **Complexity:** Low - adds early return checks for pointer equality - **Subsystem:** BPF verifier (core BPF infrastructure) - **Risk of regression:** Low - the changes are defensive checks that prevent invalid states
**Why low risk:** 1. The `reg1 == reg2` check is a simple pointer comparison 2. The logic for determining branch direction when comparing a register with itself is mathematically correct 3. The early return in `reg_set_min_max()` prevents unnecessary processing, not actual verification
### 6. USER IMPACT
**Who is affected:** - Any system running BPF programs that compare a register with itself - The triggering program is simple and can be crafted by any user with BPF access - Systems with unprivileged BPF enabled are at higher risk (denial of service)
**Severity:** - Triggers kernel BUG warning (can cause system instability) - `reg_bounds_sanity_check()` calls `verifier_bug()` which prints warnings and may affect system stability - The verifier marks the register as unbounded after the bug, which could potentially lead to incorrect verification
**Bug trigger:** The commit message shows a simple 4-instruction BPF program that triggers the bug: ``` 0: call bpf_get_prandom_u32 1: w8 = 0x80000000 2: r0 &= r8 3: if r0 > r0 goto <exit> ```
### 7. STABILITY INDICATORS
- **Tested-by:** No explicit tested-by, but tested as part of the bug report - **Reviewed/Acked-by:** Eduard Zingerman (BPF maintainer) - **Signed-off-by:** Alexei Starovoitov (BPF co-maintainer) - **Time in mainline:** This is a relatively new commit
### 8. DEPENDENCY CHECK
**Dependencies:** - The fix depends on `reg_bounds_sanity_check()` which was added in v6.8 (commit 5f99f312bd3be) - Without the sanity check, the bug would silently corrupt bounds (worse!) - The underlying bug in `is_scalar_branch_taken()` and `reg_set_min_max()` has existed since v5.0
**Affected versions:** - The visible bug (BUG warning) affects kernels ≥ v6.8 (when sanity check was added) - The silent bounds corruption affects all kernels since v5.0
### 9. BACKPORT CONSIDERATIONS
**Should be backported to:** v6.8, v6.6 (LTS), and later stable trees
**Reasons:** 1. Fixes a real, reproducible bug that triggers kernel warnings 2. Small, surgical fix with clear scope 3. Low regression risk 4. Affects core BPF infrastructure used widely 5. Easy to trigger with a simple BPF program 6. Has maintainer acknowledgment
**Backport notes:** - The patch should apply cleanly to v6.8+ where `reg_bounds_sanity_check()` exists - For v6.6 LTS, need to verify if the sanity check was backported - For older stable trees (v5.15, v5.10, etc.), the sanity check doesn't exist, so the bug manifests as silent bounds corruption which is arguably worse but harder to detect
### CONCLUSION
This commit fixes a real BPF verifier bug that: 1. Is easily reproducible with a simple BPF program 2. Triggers kernel BUG warnings (denial of service potential) 3. Has a small, well-contained fix 4. Has been acknowledged by BPF maintainers 5. Affects commonly-used infrastructure (BPF verifier)
The fix is obviously correct - comparing a register with itself should have deterministic results (e.g., r0 == r0 is always true), and adjusting bounds based on such comparisons is logically wrong.
**YES**
kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++ 1 file changed, 31 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index fbe4bb91c564a..515e4d1807c92 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -15950,6 +15950,30 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta s64 smin2 = is_jmp32 ? (s64)reg2->s32_min_value : reg2->smin_value; s64 smax2 = is_jmp32 ? (s64)reg2->s32_max_value : reg2->smax_value;
+ if (reg1 == reg2) { + switch (opcode) { + case BPF_JGE: + case BPF_JLE: + case BPF_JSGE: + case BPF_JSLE: + case BPF_JEQ: + return 1; + case BPF_JGT: + case BPF_JLT: + case BPF_JSGT: + case BPF_JSLT: + case BPF_JNE: + return 0; + case BPF_JSET: + if (tnum_is_const(t1)) + return t1.value != 0; + else + return (smin1 <= 0 && smax1 >= 0) ? -1 : 1; + default: + return -1; + } + } + switch (opcode) { case BPF_JEQ: /* constants, umin/umax and smin/smax checks would be @@ -16396,6 +16420,13 @@ static int reg_set_min_max(struct bpf_verifier_env *env, if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE) return 0;
+ /* We compute branch direction for same SCALAR_VALUE registers in + * is_scalar_branch_taken(). For unknown branch directions (e.g., BPF_JSET) + * on the same registers, we don't need to adjust the min/max values. + */ + if (false_reg1 == false_reg2) + return 0; + /* fallthrough (FALSE) branch */ regs_refine_cond_op(false_reg1, false_reg2, rev_opcode(opcode), is_jmp32); reg_bounds_sync(false_reg1);