bpf: Compare stack frames in regs_exact()

regs_exact() compares the register state up to id, followed by the ID
mappings, but does not compare frameno. The PTR_TO_STACK case in regsafe()
checks frameno separately, which is bypassed when exact comparison is
requested. Consequently, infinite-loop detection can treat pointers to
different stack frames as the same pointer and reject a finite loop.

For example, initialize fp-8 to zero in the caller and to one in the
callee, then pass the caller's fp-8 to the callee as r1:

    loop:
        r0 = *(u64 *)(r1 + 0);
        if r0 != 0 goto done;
        r1 = r10;
        r1 += -8;
        goto loop;
    done:
        exit;

The loop terminates after reading the callee's slot on its second
iteration. At the loop header, however, the only relevant difference is
r1's frameno, so exact comparison incorrectly reports an infinite loop.
The same problem occurs when the pointer is spilled to the stack.

Move frameno into the type-specific metadata union, ahead of id, so the
existing prefix comparison in regs_exact() covers it. Ordinary stack
pointers do not use another union member. Iterator and IRQ stack-slot
states use their dedicated union views and do not need a frame lookup.
This also keeps bpf_reg_state at 80 bytes.

Since frameno now shares storage with other pointer metadata, it is only
meaningful for PTR_TO_STACK registers. Return NULL from bpf_func() for
other register types. process_iter_arg(), get_constant_map_key() and
is_dynptr_reg_valid_init() look up the frame before checking the register
type and would otherwise index frame[] with a byte of the register's map
or BTF pointer. They dereference the frame only after their type check.

Move the states_maybe_looping() boundary from frameno to precise after the
field relocation. Its prefix comparison continues to cover the complete
value state and now includes frameno.

Continue to ignore precise. Precision marks control whether pruning may
ignore scalar ranges; they do not change the represented values, and exact
comparison already compares those ranges unconditionally. Marks can also
change through backtracking while an ancestor state is still being
explored.

Fixes: d5b892fd607a ("bpf: make infinite loop detection in is_state_visited() exact")
Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20260919014213.1840880-2-memxor@gmail.com

1 files changed, 15 insertions, 8 deletions

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index a7202b44ab10..6fe8e5dc57aa 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -45,6 +45,14 @@ struct bpf_reg_state {
 	union {
 		/* valid when type == PTR_TO_PACKET */
 		int range;
+		/*
+		 * Valid when type == PTR_TO_STACK. Inside the callee two registers
+		 * can be both PTR_TO_STACK like R1=fp-8 and R2=fp-8, but one of them
+		 * points to this function stack while another to the caller's stack.
+		 * To differentiate them 'frameno' is used which is an index in
+		 * bpf_verifier_state->frame[] array pointing to bpf_func_state.
+		 */
+		u8 frameno;
 
 		/*
 		 * For CONST_PTR_TO_MAP, PTR_TO_MAP_KEY, PTR_TO_MAP_VALUE and
@@ -155,14 +163,6 @@ struct bpf_reg_state {
 	 * during state comparisons.
 	 */
 	u32 map_uid;
-	/*
-	 * Inside the callee two registers can be both PTR_TO_STACK like
-	 * R1=fp-8 and R2=fp-8, but one of them points to this function stack
-	 * while another to the caller's stack. To differentiate them 'frameno'
-	 * is used which is an index in bpf_verifier_state->frame[] array
-	 * pointing to bpf_func_state.
-	 */
-	u8 frameno;
 	/* if (!precise && SCALAR_VALUE) min/max/tnum don't affect safety */
 	bool precise;
 };
@@ -1239,11 +1239,18 @@ static inline int bpf_get_spi(s32 off)
 	return (-off - 1) / BPF_REG_SIZE;
 }
 
+/*
+ * Return the function state a stack pointer register refers to. frameno
+ * shares storage with other pointer metadata, so return NULL for any
+ * other register type instead of indexing frame[] with aliased bytes.
+ */
 static inline struct bpf_func_state *bpf_func(struct bpf_verifier_env *env,
 					      const struct bpf_reg_state *reg)
 {
 	struct bpf_verifier_state *cur = env->cur_state;
 
+	if (reg->type != PTR_TO_STACK)
+		return NULL;
 	return cur->frame[reg->frameno];
 }