[PATCH 5.11 595/601] bpf: Fix alu32 const subreg bound tracking on bitwise operations

Greg Kroah-Hartman
2021-05-12T18:03:35Z

From: Daniel Borkmann <daniel@iogearbox.net>

commit 049c4e13714ecbca567b4d5f6d563f05d431c80e upstream.

Fix a bug in the verifier's scalar32_min_max_*() functions which leads to
incorrect tracking of 32 bit bounds for the simulation of and/or/xor bitops.
When both the src & dst subreg is a known constant, then the assumption is
that scalar_min_max_*() will take care to update bounds correctly. However,
this is not the case, for example, consider a register R2 which has a tnum
of 0xffffffff00000000, meaning, lower 32 bits are known constant and in this
case of value 0x00000001. R2 is then and'ed with a register R3 which is a
64 bit known constant, here, 0x100000002.

What can be seen in line '10:' is that 32 bit bounds reach an invalid state
where {u,s}32_min_value > {u,s}32_max_value. The reason is scalar32_min_max_*()
delegates 32 bit bounds updates to scalar_min_max_*(), however, that really
only takes place when both the 64 bit src & dst register is a known constant.
Given scalar32_min_max_*() is intended to be designed as closely as possible
to scalar_min_max_*(), update the 32 bit bounds in this situation through
__mark_reg32_known() which will set all {u,s}32_{min,max}_value to the correct
constant, which is 0x00000000 after the fix (given 0x00000001 & 0x00000002 in
32 bit space). This is possible given var32_off already holds the final value
as dst_reg->var_off is updated before calling scalar32_min_max_*().

Before fix, invalid tracking of R2:

[...]
9: R0_w=inv1337 R1=ctx(id=0,off=0,imm=0) R2_w=inv(id=0,smin_value=-9223372036854775807 (0x8000000000000001),smax_value=9223372032559808513 (0x7fffffff00000001),umin_value=1,umax_value=0xffffffff00000001,var_off=(0x1; 0xffffffff00000000),s32_min_value=1,s32_max_value=1,u32_min_value=1,u32_max_value=1) R3_w=inv4294967298 R10=fp0
9: (5f) r2 &= r3
10: R0_w=inv1337 R1=ctx(id=0,off=0,imm=0) R2_w=inv(id=0,smin_value=0,smax_value=4294967296 (0x100000000),umin_value=0,umax_value=0x100000000,var_off=(0x0; 0x100000000),s32_min_value=1,s32_max_value=0,u32_min_value=1,u32_max_value=0) R3_w=inv4294967298 R10=fp0
[...]

After fix, correct tracking of R2:

[...]
9: R0_w=inv1337 R1=ctx(id=0,off=0,imm=0) R2_w=inv(id=0,smin_value=-9223372036854775807 (0x8000000000000001),smax_value=9223372032559808513 (0x7fffffff00000001),umin_value=1,umax_value=0xffffffff00000001,var_off=(0x1; 0xffffffff00000000),s32_min_value=1,s32_max_value=1,u32_min_value=1,u32_max_value=1) R3_w=inv4294967298 R10=fp0
9: (5f) r2 &= r3
10: R0_w=inv1337 R1=ctx(id=0,off=0,imm=0) R2_w=inv(id=0,smin_value=0,smax_value=4294967296 (0x100000000),umin_value=0,umax_value=0x100000000,var_off=(0x0; 0x100000000),s32_min_value=0,s32_max_value=0,u32_min_value=0,u32_max_value=0) R3_w=inv4294967298 R10=fp0
[...]

Fixes: 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking")
Fixes: 2921c90d4718 ("bpf: Fix a verifier failure with xor")
Reported-by: Manfred Paul (@_manfp)
Reported-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/bpf/verifier.c | 22 +++++++++-------------
1 file changed, 9 insertions(+), 13 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6396,11 +6396,10 @@ static void scalar32_min_max_and(struct
s32 smin_val = src_reg->s32_min_value;
u32 umax_val = src_reg->u32_max_value;

- /* Assuming scalar64_min_max_and will be called so its safe
- * to skip updating register for known 32-bit case.
- */
- if (src_known && dst_known)
+ if (src_known && dst_known) {
+ __mark_reg32_known(dst_reg, var32_off.value);
return;
+ }

/* We get our minimum from the var_off, since that's inherently
* bitwise. Our maximum is the minimum of the operands' maxima.
@@ -6420,7 +6419,6 @@ static void scalar32_min_max_and(struct
dst_reg->s32_min_value = dst_reg->u32_min_value;
dst_reg->s32_max_value = dst_reg->u32_max_value;
}
-
}

static void scalar_min_max_and(struct bpf_reg_state *dst_reg,
@@ -6467,11 +6465,10 @@ static void scalar32_min_max_or(struct b
s32 smin_val = src_reg->s32_min_value;
u32 umin_val = src_reg->u32_min_value;

- /* Assuming scalar64_min_max_or will be called so it is safe
- * to skip updating register for known case.
- */
- if (src_known && dst_known)
+ if (src_known && dst_known) {
+ __mark_reg32_known(dst_reg, var32_off.value);
return;
+ }

/* We get our maximum from the var_off, and our minimum is the
* maximum of the operands' minima
@@ -6536,11 +6533,10 @@ static void scalar32_min_max_xor(struct
struct tnum var32_off = tnum_subreg(dst_reg->var_off);
s32 smin_val = src_reg->s32_min_value;

- /* Assuming scalar64_min_max_xor will be called so it is safe
- * to skip updating register for known case.
- */
- if (src_known && dst_known)
+ if (src_known && dst_known) {
+ __mark_reg32_known(dst_reg, var32_off.value);
return;
+ }

/* We get both minimum and maximum from the var32_off. */
dst_reg->u32_min_value = var32_off.value;