From: Andrey Ignatov rdna@fb.com
commit 088ec26d9c2da9d879ab73e3f4117f9df6c566ee upstream.
Proper support of indirect stack access with variable offset in unprivileged mode (!root) requires corresponding support in Spectre masking for stack ALU in retrieve_ptr_limit().
There are no use-case for variable offset in unprivileged mode though so make verifier reject such accesses for simplicity.
Pointer arithmetics is one (and only?) way to cause variable offset and it's already rejected in unpriv mode so that verifier won't even get to helper function whose argument contains variable offset, e.g.:
0: (7a) *(u64 *)(r10 -16) = 0 1: (7a) *(u64 *)(r10 -8) = 0 2: (61) r2 = *(u32 *)(r1 +0) 3: (57) r2 &= 4 4: (17) r2 -= 16 5: (0f) r2 += r10 variable stack access var_off=(0xfffffffffffffff0; 0x4) off=-16 size=1R2 stack pointer arithmetic goes out of range, prohibited for !root
Still it looks like a good idea to reject variable offset indirect stack access for unprivileged mode in check_stack_boundary() explicitly.
Fixes: 2011fccfb61b ("bpf: Support variable offset stack access from helpers") Reported-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Andrey Ignatov rdna@fb.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net [OP: drop comment in retrieve_ptr_limit()] Signed-off-by: Ovidiu Panait ovidiu.panait@windriver.com --- kernel/bpf/verifier.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index a5360b603e4c..d2f3bbff3175 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1811,6 +1811,19 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno, if (err) return err; } else { + /* Variable offset is prohibited for unprivileged mode for + * simplicity since it requires corresponding support in + * Spectre masking for stack ALU. + * See also retrieve_ptr_limit(). + */ + if (!env->allow_ptr_leaks) { + char tn_buf[48]; + + tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off); + verbose(env, "R%d indirect variable offset stack access prohibited for !root, var_off=%s\n", + regno, tn_buf); + return -EACCES; + } /* Only initialized buffer on stack is allowed to be accessed * with variable offset. With uninitialized buffer it's hard to * guarantee that whole memory is marked as initialized on