Patch "bpf, array: fix overflow in max_entries and undefined behavior in index_mask" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
bpf-array-fix-overflow-in-max_entries-and-undefined-behavior-in-index_mask.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Daniel Borkmann <[email protected]>
Date: Wed, 10 Jan 2018 23:25:05 +0100
Subject: bpf, array: fix overflow in max_entries and undefined behavior in index_mask

From: Daniel Borkmann <[email protected]>

commit bbeb6e4323dad9b5e0ee9f60c223dd532e2403b1 upstream.

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.

---
 kernel/bpf/arraymap.c |   18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

@@ -53,7 +53,7 @@ static struct bpf_map *array_map_alloc(u
 	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 ||

@@ -70,13 +70,25 @@ static struct bpf_map *array_map_alloc(u
 	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)

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/bpf-array-fix-overflow-in-max_entries-and-undefined-behavior-in-index_mask.patch queue-4.14/bpf-prevent-out-of-bounds-speculation.patch queue-4.14/bpf-arsh-is-not-supported-in-32-bit-alu-thus-reject-it.patch