bpf: Protect against int overflow for stack access size

[ Upstream commit ecc6a2101840177e57c925c102d2d29f260d37c8 ]

This patch re-introduces protection against the size of access to stack
memory being negative; the access size can appear negative as a result
of overflowing its signed int representation. This should not actually
happen, as there are other protections along the way, but we should
protect against it anyway. One code path was missing such protections
(fixed in the previous patch in the series), causing out-of-bounds array
accesses in check_stack_range_initialized(). This patch causes the
verification of a program with such a non-sensical access size to fail.

This check used to exist in a more indirect way, but was inadvertendly
removed in a833a17aeac7.

Fixes: a833a17aeac7 ("bpf: Fix verification of indirect var-off stack access")
Reported-by: syzbot+33f4297b5f927648741a@syzkaller.appspotmail.com
Reported-by: syzbot+aafd0513053a1cbf52ef@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/bpf/CAADnVQLORV5PT0iTAhRER+iLBTkByCYNBYyvBSgjN1T31K+gOw@mail.gmail.com/
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Link: https://lore.kernel.org/r/20240327024245.318299-3-andreimatei1@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index f099c5481b662556ac6cc6a9b16ede59ef387a19..008ddb694c8a1106369eedf5e3d485d95f79a5a4 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4320,6 +4320,11 @@ static int check_stack_access_within_bounds(
        err = check_stack_slot_within_bounds(min_off, state, type);
        if (!err && max_off > 0)
                err = -EINVAL; /* out of stack access into non-negative offsets */
+       if (!err && access_size < 0)
+               /* access_size should not be negative (or overflow an int); others checks
+                * along the way should have prevented such an access.
+                */
+               err = -EFAULT; /* invalid negative access size; integer overflow? */
 
        if (err) {
                if (tnum_is_const(reg->var_off)) {