| Message ID | 20180110222505.4845-1-daniel@iogearbox.net |
|---|---|
| State | Accepted, archived |
| Delegated to: | BPF Maintainers |
| Headers | show |
| Series | [bpf] bpf, array: fix overflow in max_entries and undefined behavior in index_mask | expand |
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.
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)
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(-)