[PATCH bpf] bpf, array: fix overflow in max_entries and undefined behavior in index_mask

[PATCH bpf] bpf, array: fix overflow in max_entries and undefined behavior in index_mask

[Daniel Borkmann]

syzkaller tried to alloc a map with 0xfffffffd entries out of a userns,
and thus unprivileged. With the recently added logic in b2157399cc98
("bpf: prevent out-of-bounds speculation") we round this up to the next
power of two value for max_entries for unprivileged such that we can
apply proper masking into potentially zeroed out map slots.

However, this will generate an index_mask of 0xffffffff, and therefore
a + 1 will let this overflow into new max_entries of 0. This will pass
allocation, etc, and later on map access we still enforce on the original
attr->max_entries value which was 0xfffffffd, therefore triggering GPF
all over the place. Thus bail out on overflow in such case.

Moreover, on 32 bit archs roundup_pow_of_two() can also not be used,
since fls_long(max_entries - 1) can result in 32 and 1UL << 32 in 32 bit
space is undefined. Therefore, do this by hand in a 64 bit variable.

This fixes all the issues triggered by syzkaller's reproducers.

Fixes: b2157399cc98 ("bpf: prevent out-of-bounds speculation")
Reported-by: syzbot+b0efb8e572d01bce1ae0@syzkaller.appspotmail.com
Reported-by: syzbot+6c15e9744f75f2364773@syzkaller.appspotmail.com
Reported-by: syzbot+d2f5524fb46fd3b312ee@syzkaller.appspotmail.com
Reported-by: syzbot+61d23c95395cc90dbc2b@syzkaller.appspotmail.com
Reported-by: syzbot+0d363c942452cca68c01@syzkaller.appspotmail.com
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 kernel/bpf/arraymap.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index aaa3198..ab94d30 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -56,7 +56,7 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
 	u32 elem_size, index_mask, max_entries;
 	bool unpriv = !capable(CAP_SYS_ADMIN);
 	struct bpf_array *array;
-	u64 array_size;
+	u64 array_size, mask64;
 
 	/* check sanity of attributes */
 	if (attr->max_entries == 0 || attr->key_size != 4 ||
@@ -74,13 +74,25 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
 	elem_size = round_up(attr->value_size, 8);
 
 	max_entries = attr->max_entries;
-	index_mask = roundup_pow_of_two(max_entries) - 1;
 
-	if (unpriv)
+	/* On 32 bit archs roundup_pow_of_two() with max_entries that has
+	 * upper most bit set in u32 space is undefined behavior due to
+	 * resulting 1U << 32, so do it manually here in u64 space.
+	 */
+	mask64 = fls_long(max_entries - 1);
+	mask64 = 1ULL << mask64;
+	mask64 -= 1;
+
+	index_mask = mask64;
+	if (unpriv) {
 		/* round up array size to nearest power of 2,
 		 * since cpu will speculate within index_mask limits
 		 */
 		max_entries = index_mask + 1;
+		/* Check for overflows. */
+		if (max_entries < attr->max_entries)
+			return ERR_PTR(-E2BIG);
+	}
 
 	array_size = sizeof(*array);
 	if (percpu)
-- 
2.9.5

Re: [PATCH bpf] bpf, array: fix overflow in max_entries and undefined behavior in index_mask

[Alexei Starovoitov]

On Wed, Jan 10, 2018 at 11:25:05PM +0100, Daniel Borkmann wrote:
> syzkaller tried to alloc a map with 0xfffffffd entries out of a userns,
> and thus unprivileged. With the recently added logic in b2157399cc98
> ("bpf: prevent out-of-bounds speculation") we round this up to the next
> power of two value for max_entries for unprivileged such that we can
> apply proper masking into potentially zeroed out map slots.
> 
> However, this will generate an index_mask of 0xffffffff, and therefore
> a + 1 will let this overflow into new max_entries of 0. This will pass
> allocation, etc, and later on map access we still enforce on the original
> attr->max_entries value which was 0xfffffffd, therefore triggering GPF
> all over the place. Thus bail out on overflow in such case.
> 
> Moreover, on 32 bit archs roundup_pow_of_two() can also not be used,
> since fls_long(max_entries - 1) can result in 32 and 1UL << 32 in 32 bit
> space is undefined. Therefore, do this by hand in a 64 bit variable.
> 
> This fixes all the issues triggered by syzkaller's reproducers.
> 
> Fixes: b2157399cc98 ("bpf: prevent out-of-bounds speculation")
> Reported-by: syzbot+b0efb8e572d01bce1ae0@syzkaller.appspotmail.com
> Reported-by: syzbot+6c15e9744f75f2364773@syzkaller.appspotmail.com
> Reported-by: syzbot+d2f5524fb46fd3b312ee@syzkaller.appspotmail.com
> Reported-by: syzbot+61d23c95395cc90dbc2b@syzkaller.appspotmail.com
> Reported-by: syzbot+0d363c942452cca68c01@syzkaller.appspotmail.com
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Applied, thank you Daniel.