Commit 69772f50 authored Dec 03, 2024 by Kumar Kartikeya Dwivedi Committed by Alexei Starovoitov Dec 04, 2024

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: eaf18feb ("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>

parent 5a6ea702

kernel/bpf/verifier.c

+6 −3

Original line number	Diff line number	Diff line
		@@ -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;
		}