Patch "bpf: mark dst unknown on inconsistent {s, u}bounds adjustments" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

bpf: mark dst unknown on inconsistent {s, u}bounds adjustments

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
bpf-mark-dst-unknown-on-inconsistent-s-u-bounds-adjustments.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Daniel Borkmann <[email protected]>
Date: Thu, 18 Jan 2018 01:15:21 +0100
Subject: bpf: mark dst unknown on inconsistent {s, u}bounds adjustments

From: Daniel Borkmann <[email protected]>

commit 6f16101e6a8b4324c36e58a29d9e0dbb287cdedb upstream.

syzkaller generated a BPF proglet and triggered a warning with the following:

0: (b7) r0 = 0
1: (d5) if r0 s<= 0x0 goto pc+0
R0=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0
2: (1f) r0 -= r1
R0=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0
verifier internal error: known but bad sbounds

What happens is that in the first insn, r0's min/max value are both 0 due to the immediate assignment, later in the jsle test the bounds are updated for the min value in the false path, meaning, they yield smin_val = 1, smax_val = 0, and when ctx pointer is subtracted from r0, verifier bails out with the internal error and throwing a WARN since smin_val != smax_val for the known constant.

For min_val > max_val scenario it means that reg_set_min_max() and reg_set_min_max_inv() (which both refine existing bounds) demonstrated that such branch cannot be taken at runtime.

In above scenario for the case where it will be taken, the existing [0, 0] bounds are kept intact. Meaning, the rejection is not due to a verifier internal error, and therefore the WARN() is not necessary either.

We could just reject such cases in adjust_{ptr,scalar}_min_max_vals() when either known scalars have smin_val != smax_val or umin_val != umax_val or any scalar reg with bounds smin_val > smax_val or umin_val > umax_val. However, there may be a small risk of breakage of buggy programs, so handle this more gracefully and in adjust_{ptr,scalar}_min_max_vals() just taint the dst reg as unknown scalar when we see ops with such kind of src reg.

---
 kernel/bpf/verifier.c                       |   25 +++--
 tools/testing/selftests/bpf/test_verifier.c |  123 +++++++++++++++++++++++++++-
 2 files changed, 138 insertions(+), 10 deletions(-)

@@ -1865,15 +1865,13 @@ static int adjust_ptr_min_max_vals(struc
 
 	dst_reg = &regs[dst];
 
-	if (WARN_ON_ONCE(known && (smin_val != smax_val))) {
-		print_verifier_state(&env->cur_state);
-		verbose("verifier internal error: known but bad sbounds\n");
-		return -EINVAL;
-	}
-	if (WARN_ON_ONCE(known && (umin_val != umax_val))) {
-		print_verifier_state(&env->cur_state);
-		verbose("verifier internal error: known but bad ubounds\n");
-		return -EINVAL;
+	if ((known && (smin_val != smax_val || umin_val != umax_val)) ||
+	    smin_val > smax_val || umin_val > umax_val) {
+		/* Taint dst register if offset had invalid bounds derived from
+		 * e.g. dead branches.
+		 */
+		__mark_reg_unknown(dst_reg);
+		return 0;
 	}
 
 	if (BPF_CLASS(insn->code) != BPF_ALU64) {

@@ -2075,6 +2073,15 @@ static int adjust_scalar_min_max_vals(st
 	src_known = tnum_is_const(src_reg.var_off);
 	dst_known = tnum_is_const(dst_reg->var_off);
 
+	if ((src_known && (smin_val != smax_val || umin_val != umax_val)) ||
+	    smin_val > smax_val || umin_val > umax_val) {
+		/* Taint dst register if offset had invalid bounds derived from
+		 * e.g. dead branches.
+		 */
+		__mark_reg_unknown(dst_reg);
+		return 0;
+	}
+
 	if (!src_known &&
 	    opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) {
 		__mark_reg_unknown(dst_reg);

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/bpf-mark-dst-unknown-on-inconsistent-s-u-bounds-adjustments.patch