Thanks for spotting this - does anyone remember when this version of
find_next bitops was first introduced?
I wonder whether this might explain our crashes while using SLUB in
recent kernels.
Michael
On Tue, Feb 8, 2011 at 1:37 AM, Akinobu Mita <akinobu.mita@xxxxxxxxx> wrote:
find_next bitops on m68k (find_next_zero_bit, find_next_bit, and
ext2_find_next_bit) may cause out of bounds memory access
when the bitmap size in bits % 32 != 0 and offset (the bitnumber
to start searching at) is very close to the bitmap size.
For example,
unsigned long bitmap[2] = { 0, 0};
find_next_bit(bitmap, 63, 62)
1. find_next_bit() tries to find any set bits in bitmap[1], but no bits set
2. Then find_first_bit(bimap + 2, -1)
3. Unfortunately find_fist_bit() takes unsigned int as the size argument.
Can't that be changed? After all, find_next_bit takes int as size as well.
4. find_first_bit will access bitmap[3~] until it find any set bits.
Here is find_next_bit() in arch/m68k/include/asm/bitops_mm.h:
static inline int find_next_bit(const unsigned long *vaddr, int size,
int offset)
{
const unsigned long *p = vaddr + (offset >> 5);
int bit = offset & 31UL, res;
if (offset >= size)
return size;
if (bit) {
unsigned long num = *p++ & (~0UL << bit);
offset -= bit;
/* Look for one in first longword */
__asm__ __volatile__ ("bfffo %1{#0,#0},%0"
: "=d" (res) : "d" (num & -num));
if (res < 32)
return offset + (res ^ 31);
offset += 32;
}
/* No one yet, search remaining full bytes for a one */
I'll try adding a sanity check here to see whether this actually
causes any trouble with SLUB.
res = find_first_bit(p, size - ((long)p - (long)vaddr) * 8);
return offset + res;
}
find_next_zero_bit and ext2_find_next_bit also have same problem.
Actually I found this while testing ext2_find_next_bit in user space.
The easiest and reliable way to fix this is that switching to generic
implementation of find bitops.
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