CVE-2020-27194: Incorrect Conversion between Numeric Types (CWE-681)
value: An issue was discovered in the Linux kernel before 5.8.15. scalar32_min_max_or in kernel/bpf/verifier.c mishandles bounds tracking during use of 64-bit values, aka CID-5b9fbeb75b6a.
CWE description: When converting from one data type to another, such as long to integer, data can be omitted or translated in a way that produces unexpected values. If the resulting values are used in a sensitive context, then dangerous behaviors may occur.
CVSS v3 base score: 5.5
Published: 2020-10-16T21:15Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: kernel/bpf/verifier.c
Commit date: 2020-10-08 11:02:53 +0200
Commit message: bpf: Fix scalar32_min_max_or bounds tracking

Simon reported an issue with the current scalar32_min_max_or() implementation.
That is, compared to the other 32 bit subreg tracking functions, the code in
scalar32_min_max_or() stands out that it's using the 64 bit registers instead
of 32 bit ones. This leads to bounds tracking issues, for example:

  [...]
  8: R0=map_value(id=0,off=0,ks=4,vs=48,imm=0) R10=fp0 fp-8=mmmmmmmm
  8: (79) r1 = *(u64 *)(r0 +0)
   R0=map_value(id=0,off=0,ks=4,vs=48,imm=0) R10=fp0 fp-8=mmmmmmmm
  9: R0=map_value(id=0,off=0,ks=4,vs=48,imm=0) R1_w=inv(id=0) R10=fp0 fp-8=mmmmmmmm
  9: (b7) r0 = 1
  10: R0_w=inv1 R1_w=inv(id=0) R10=fp0 fp-8=mmmmmmmm
  10: (18) r2 = 0x600000002
  12: R0_w=inv1 R1_w=inv(id=0) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  12: (ad) if r1 < r2 goto pc+1
   R0_w=inv1 R1_w=inv(id=0,umin_value=25769803778) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  13: R0_w=inv1 R1_w=inv(id=0,umin_value=25769803778) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  13: (95) exit
  14: R0_w=inv1 R1_w=inv(id=0,umax_value=25769803777,var_off=(0x0; 0x7ffffffff)) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  14: (25) if r1 > 0x0 goto pc+1
   R0_w=inv1 R1_w=inv(id=0,umax_value=0,var_off=(0x0; 0x7fffffff),u32_max_value=2147483647) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  15: R0_w=inv1 R1_w=inv(id=0,umax_value=0,var_off=(0x0; 0x7fffffff),u32_max_value=2147483647) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  15: (95) exit
  16: R0_w=inv1 R1_w=inv(id=0,umin_value=1,umax_value=25769803777,var_off=(0x0; 0x77fffffff),u32_max_value=2147483647) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  16: (47) r1 |= 0
  17: R0_w=inv1 R1_w=inv(id=0,umin_value=1,umax_value=32212254719,var_off=(0x1; 0x700000000),s32_max_value=1,u32_max_value=1) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  [...]

The bound tests on the map value force the upper unsigned bound to be 25769803777
in 64 bit (0b11000000000000000000000000000000001) and then lower one to be 1. By
using OR they are truncated and thus result in the range [1,1] for the 32 bit reg
tracker. This is incorrect given the only thing we know is that the value must be
positive and thus 2147483647 (0b1111111111111111111111111111111) at max for the
subregs. Fix it by using the {u,s}32_{min,max}_value vars instead. This also makes
sense, for example, for the case where we update dst_reg->s32_{min,max}_value in
the else branch we need to use the newly computed dst_reg->u32_{min,max}_value as
we know that these are positive. Previously, in the else branch the 64 bit values
of umin_value=1 and umax_value=32212254719 were used and latter got truncated to
be 1 as upper bound there. After the fix the subreg range is now correct:

  [...]
  8: R0=map_value(id=0,off=0,ks=4,vs=48,imm=0) R10=fp0 fp-8=mmmmmmmm
  8: (79) r1 = *(u64 *)(r0 +0)
   R0=map_value(id=0,off=0,ks=4,vs=48,imm=0) R10=fp0 fp-8=mmmmmmmm
  9: R0=map_value(id=0,off=0,ks=4,vs=48,imm=0) R1_w=inv(id=0) R10=fp0 fp-8=mmmmmmmm
  9: (b7) r0 = 1
  10: R0_w=inv1 R1_w=inv(id=0) R10=fp0 fp-8=mmmmmmmm
  10: (18) r2 = 0x600000002
  12: R0_w=inv1 R1_w=inv(id=0) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  12: (ad) if r1 < r2 goto pc+1
   R0_w=inv1 R1_w=inv(id=0,umin_value=25769803778) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  13: R0_w=inv1 R1_w=inv(id=0,umin_value=25769803778) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  13: (95) exit
  14: R0_w=inv1 R1_w=inv(id=0,umax_value=25769803777,var_off=(0x0; 0x7ffffffff)) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  14: (25) if r1 > 0x0 goto pc+1
   R0_w=inv1 R1_w=inv(id=0,umax_value=0,var_off=(0x0; 0x7fffffff),u32_max_value=2147483647) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  15: R0_w=inv1 R1_w=inv(id=0,umax_value=0,var_off=(0x0; 0x7fffffff),u32_max_value=2147483647) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  15: (95) exit
  16: R0_w=inv1 R1_w=inv(id=0,umin_value=1,umax_value=25769803777,var_off=(0x0; 0x77fffffff),u32_max_value=2147483647) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  16: (47) r1 |= 0
  17: R0_w=inv1 R1_w=inv(id=0,umin_value=1,umax_value=32212254719,var_off=(0x0; 0x77fffffff),u32_max_value=2147483647) R2_w=inv25769803778 R10=fp0 fp-8=mmmmmmmm
  [...]

Fixes: 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking")
Reported-by: Simon Scannell <scannell.smn@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>

Diff: diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 47e74f09fa37..fba52d9ec8fc 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5665,12 +5665,12 @@ static void scalar32_min_max_or(struct bpf_reg_state *dst_reg,
 				struct bpf_reg_state *src_reg)
 {
 	bool src_known = tnum_subreg_is_const(src_reg->var_off);
 	bool dst_known = tnum_subreg_is_const(dst_reg->var_off);
 	struct tnum var32_off = tnum_subreg(dst_reg->var_off);
-	s32 smin_val = src_reg->smin_value;
-	u32 umin_val = src_reg->umin_value;
+	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)
@@ -5689,12 +5689,12 @@ static void scalar32_min_max_or(struct bpf_reg_state *dst_reg,
 		dst_reg->s32_max_value = S32_MAX;
 	} else {
 		/* ORing two positives gives a positive, so safe to
 		 * cast result into s64.
 		 */
-		dst_reg->s32_min_value = dst_reg->umin_value;
-		dst_reg->s32_max_value = dst_reg->umax_value;
+		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_or(struct bpf_reg_state *dst_reg,
 			      struct bpf_reg_state *src_reg)

Vulnerable code: 	s32 smin_val = src_reg->smin_value;
	u32 umin_val = src_reg->umin_value;
		dst_reg->s32_min_value = dst_reg->umin_value;
		dst_reg->s32_max_value = dst_reg->umax_value;
Fixed code: 	s32 smin_val = src_reg->s32_min_value;
	u32 umin_val = src_reg->u32_min_value;
		dst_reg->s32_min_value = dst_reg->u32_min_value;
		dst_reg->s32_max_value = dst_reg->u32_max_value;