bpf: Don't mark STACK_INVALID as STACK_MISC in mark_stack_slot_misc

Inside mark_stack_slot_misc, we should not upgrade STACK_INVALID to
STACK_MISC when allow_ptr_leaks is false, since invalid contents
shouldn't be read unless the program has the relevant capabilities.
The relaxation only makes sense when env->allow_ptr_leaks is true.

However, such conversion in privileged mode becomes unnecessary, as
invalid slots can be read without being upgraded to STACK_MISC.

Currently, the condition is inverted (i.e. checking for true instead of
false), simply remove it to restore correct behavior.

Fixes: eaf18febd6eb ("bpf: preserve STACK_ZERO slots on partial reg spills")
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Reported-by: Tao Lyu <tao.lyu@epfl.ch>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241204044757.1483141-2-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2fd35465d650a0338a58ffe87d8d8ea085910c38..f18aad339de878c9379aaa9bfb4c1a054d6faf89 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1202,14 +1202,17 @@ static bool is_spilled_scalar_reg64(const struct bpf_stack_state *stack)
 /* Mark stack slot as STACK_MISC, unless it is already STACK_INVALID, in which
  * case they are equivalent, or it's STACK_ZERO, in which case we preserve
  * more precise STACK_ZERO.
- * Note, in uprivileged mode leaving STACK_INVALID is wrong, so we take
- * env->allow_ptr_leaks into account and force STACK_MISC, if necessary.
+ * Regardless of allow_ptr_leaks setting (i.e., privileged or unprivileged
+ * mode), we won't promote STACK_INVALID to STACK_MISC. In privileged case it is
+ * unnecessary as both are considered equivalent when loading data and pruning,
+ * in case of unprivileged mode it will be incorrect to allow reads of invalid
+ * slots.
  */
 static void mark_stack_slot_misc(struct bpf_verifier_env *env, u8 *stype)
 {
        if (*stype == STACK_ZERO)
                return;
-       if (env->allow_ptr_leaks && *stype == STACK_INVALID)
+       if (*stype == STACK_INVALID)
                return;
        *stype = STACK_MISC;
 }