netdev - [PATCH 4.4-stable 7/7] bpf, array: fix overflow in max_entries and undefined behavior in index

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Date:   Fri, 12 Jan 2018 17:58:05 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     gregkh@...uxfoundation.org
Cc:     stable@...r.kernel.org, ast@...nel.org, netdev@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 4.4-stable 7/7] bpf, array: fix overflow in max_entries and undefined behavior in index_mask

From: Daniel Borkmann <daniel@...earbox.net>

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.

Fixes: b2157399cc98 ("bpf: prevent out-of-bounds speculation")
Reported-by: syzbot+b0efb8e572d01bce1ae0@...kaller.appspotmail.com
Reported-by: syzbot+6c15e9744f75f2364773@...kaller.appspotmail.com
Reported-by: syzbot+d2f5524fb46fd3b312ee@...kaller.appspotmail.com
Reported-by: syzbot+61d23c95395cc90dbc2b@...kaller.appspotmail.com
Reported-by: syzbot+0d363c942452cca68c01@...kaller.appspotmail.com
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Signed-off-by: Alexei Starovoitov <ast@...nel.org>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 kernel/bpf/arraymap.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index 56f8a8306a49..3608fa1aec8a 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -23,6 +23,7 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
 	u32 elem_size, array_size, index_mask, max_entries;
 	bool unpriv = !capable(CAP_SYS_ADMIN);
 	struct bpf_array *array;
+	u64 mask64;
 
 	/* check sanity of attributes */
 	if (attr->max_entries == 0 || attr->key_size != 4 ||
@@ -38,13 +39,25 @@ 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);
+	}
 
 	/* check round_up into zero and u32 overflow */
 	if (elem_size == 0 ||
-- 
2.15.1