From: Jann Horn jannh@google.com
[ Upstream commit 604dca5e3af1db98bd123b7bfc02b017af99e3a0 ]
The BPF verifier tried to track values based on 32-bit comparisons by (ab)using the tnum state via 581738a681b6 ("bpf: Provide better register bounds after jmp32 instructions"). The idea is that after a check like this:
if ((u32)r0 > 3) exit
We can't meaningfully constrain the arithmetic-range-based tracking, but we can update the tnum state to (value=0,mask=0xffff'ffff'0000'0003). However, the implementation from 581738a681b6 didn't compute the tnum constraint based on the fixed operand, but instead derives it from the arithmetic-range-based tracking. This means that after the following sequence of operations:
if (r0 >= 0x1'0000'0001) exit if ((u32)r0 > 7) exit
The verifier assumed that the lower half of r0 is in the range (0, 0) and apply the tnum constraint (value=0,mask=0xffff'ffff'0000'0000) thus causing the overall tnum to be (value=0,mask=0x1'0000'0000), which was incorrect. Provide a fixed implementation.
Fixes: 581738a681b6 ("bpf: Provide better register bounds after jmp32 instructions") Signed-off-by: Jann Horn jannh@google.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Alexei Starovoitov ast@kernel.org Link: https://lore.kernel.org/bpf/20200330160324.15259-3-daniel@iogearbox.net Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/verifier.c | 108 ++++++++++++++++++++++++++++-------------- 1 file changed, 72 insertions(+), 36 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 5080469094afe..595b39eee6422 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -5590,6 +5590,70 @@ static bool cmp_val_with_extended_s64(s64 sval, struct bpf_reg_state *reg) reg->smax_value <= 0 && reg->smin_value >= S32_MIN); }
+/* Constrain the possible values of @reg with unsigned upper bound @bound. + * If @is_exclusive, @bound is an exclusive limit, otherwise it is inclusive. + * If @is_jmp32, @bound is a 32-bit value that only constrains the low 32 bits + * of @reg. + */ +static void set_upper_bound(struct bpf_reg_state *reg, u64 bound, bool is_jmp32, + bool is_exclusive) +{ + if (is_exclusive) { + /* There are no values for `reg` that make `reg<0` true. */ + if (bound == 0) + return; + bound--; + } + if (is_jmp32) { + /* Constrain the register's value in the tnum representation. + * For 64-bit comparisons this happens later in + * __reg_bound_offset(), but for 32-bit comparisons, we can be + * more precise than what can be derived from the updated + * numeric bounds. + */ + struct tnum t = tnum_range(0, bound); + + t.mask |= ~0xffffffffULL; /* upper half is unknown */ + reg->var_off = tnum_intersect(reg->var_off, t); + + /* Compute the 64-bit bound from the 32-bit bound. */ + bound += gen_hi_max(reg->var_off); + } + reg->umax_value = min(reg->umax_value, bound); +} + +/* Constrain the possible values of @reg with unsigned lower bound @bound. + * If @is_exclusive, @bound is an exclusive limit, otherwise it is inclusive. + * If @is_jmp32, @bound is a 32-bit value that only constrains the low 32 bits + * of @reg. + */ +static void set_lower_bound(struct bpf_reg_state *reg, u64 bound, bool is_jmp32, + bool is_exclusive) +{ + if (is_exclusive) { + /* There are no values for `reg` that make `reg>MAX` true. */ + if (bound == (is_jmp32 ? U32_MAX : U64_MAX)) + return; + bound++; + } + if (is_jmp32) { + /* Constrain the register's value in the tnum representation. + * For 64-bit comparisons this happens later in + * __reg_bound_offset(), but for 32-bit comparisons, we can be + * more precise than what can be derived from the updated + * numeric bounds. + */ + struct tnum t = tnum_range(bound, U32_MAX); + + t.mask |= ~0xffffffffULL; /* upper half is unknown */ + reg->var_off = tnum_intersect(reg->var_off, t); + + /* Compute the 64-bit bound from the 32-bit bound. */ + bound += gen_hi_min(reg->var_off); + } + reg->umin_value = max(reg->umin_value, bound); +} + /* Adjusts the register min/max values in the case that the dst_reg is the * variable register that we are working on, and src_reg is a constant or we're * simply doing a BPF_K check. @@ -5645,15 +5709,8 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg, case BPF_JGE: case BPF_JGT: { - u64 false_umax = opcode == BPF_JGT ? val : val - 1; - u64 true_umin = opcode == BPF_JGT ? val + 1 : val; - - if (is_jmp32) { - false_umax += gen_hi_max(false_reg->var_off); - true_umin += gen_hi_min(true_reg->var_off); - } - false_reg->umax_value = min(false_reg->umax_value, false_umax); - true_reg->umin_value = max(true_reg->umin_value, true_umin); + set_upper_bound(false_reg, val, is_jmp32, opcode == BPF_JGE); + set_lower_bound(true_reg, val, is_jmp32, opcode == BPF_JGT); break; } case BPF_JSGE: @@ -5674,15 +5731,8 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg, case BPF_JLE: case BPF_JLT: { - u64 false_umin = opcode == BPF_JLT ? val : val + 1; - u64 true_umax = opcode == BPF_JLT ? val - 1 : val; - - if (is_jmp32) { - false_umin += gen_hi_min(false_reg->var_off); - true_umax += gen_hi_max(true_reg->var_off); - } - false_reg->umin_value = max(false_reg->umin_value, false_umin); - true_reg->umax_value = min(true_reg->umax_value, true_umax); + set_lower_bound(false_reg, val, is_jmp32, opcode == BPF_JLE); + set_upper_bound(true_reg, val, is_jmp32, opcode == BPF_JLT); break; } case BPF_JSLE: @@ -5757,15 +5807,8 @@ static void reg_set_min_max_inv(struct bpf_reg_state *true_reg, case BPF_JGE: case BPF_JGT: { - u64 false_umin = opcode == BPF_JGT ? val : val + 1; - u64 true_umax = opcode == BPF_JGT ? val - 1 : val; - - if (is_jmp32) { - false_umin += gen_hi_min(false_reg->var_off); - true_umax += gen_hi_max(true_reg->var_off); - } - false_reg->umin_value = max(false_reg->umin_value, false_umin); - true_reg->umax_value = min(true_reg->umax_value, true_umax); + set_lower_bound(false_reg, val, is_jmp32, opcode == BPF_JGE); + set_upper_bound(true_reg, val, is_jmp32, opcode == BPF_JGT); break; } case BPF_JSGE: @@ -5783,15 +5826,8 @@ static void reg_set_min_max_inv(struct bpf_reg_state *true_reg, case BPF_JLE: case BPF_JLT: { - u64 false_umax = opcode == BPF_JLT ? val : val - 1; - u64 true_umin = opcode == BPF_JLT ? val + 1 : val; - - if (is_jmp32) { - false_umax += gen_hi_max(false_reg->var_off); - true_umin += gen_hi_min(true_reg->var_off); - } - false_reg->umax_value = min(false_reg->umax_value, false_umax); - true_reg->umin_value = max(true_reg->umin_value, true_umin); + set_upper_bound(false_reg, val, is_jmp32, opcode == BPF_JLE); + set_lower_bound(true_reg, val, is_jmp32, opcode == BPF_JLT); break; } case BPF_JSLE: