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bpf: Use __u64 to save the bits in bits iterator

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On 32-bit hosts (e.g., arm32), when a bpf program passes a u64 to
bpf_iter_bits_new(), bpf_iter_bits_new() will use bits_copy to store the
content of the u64. However, bits_copy is only 4 bytes, leading to stack
corruption.

The straightforward solution would be to replace u64 with unsigned long
in bpf_iter_bits_new(). However, this introduces confusion and problems
for 32-bit hosts because the size of ulong in bpf program is 8 bytes,
but it is treated as 4-bytes after passed to bpf_iter_bits_new().

Fix it by changing the type of both bits and bit_count from unsigned
long to u64. However, the change is not enough. The main reason is that
bpf_iter_bits_next() uses find_next_bit() to find the next bit and the
pointer passed to find_next_bit() is an unsigned long pointer instead
of a u64 pointer. For 32-bit little-endian host, it is fine but it is
not the case for 32-bit big-endian host. Because under 32-bit big-endian
host, the first iterated unsigned long will be the bits 32-63 of the u64
instead of the expected bits 0-31. Therefore, in addition to changing
the type, swap the two unsigned longs within the u64 for 32-bit
big-endian host.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20241030100516.3633640-5-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Hou Tao 2024-10-30 18:05:15 +08:00 committed by Alexei Starovoitov
parent 393397fbdc
commit e133938367
1 changed files with 30 additions and 3 deletions
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33
kernel/bpf/helpers.c
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@ -2855,13 +2855,36 @@ struct bpf_iter_bits {
struct bpf_iter_bits_kern {
union {
unsigned long *bits;
unsigned long bits_copy;
__u64 *bits;
__u64 bits_copy;
};
int nr_bits;
int bit;
} __aligned(8);
/* On 64-bit hosts, unsigned long and u64 have the same size, so passing
* a u64 pointer and an unsigned long pointer to find_next_bit() will
* return the same result, as both point to the same 8-byte area.
*
* For 32-bit little-endian hosts, using a u64 pointer or unsigned long
* pointer also makes no difference. This is because the first iterated
* unsigned long is composed of bits 0-31 of the u64 and the second unsigned
* long is composed of bits 32-63 of the u64.
*
* However, for 32-bit big-endian hosts, this is not the case. The first
* iterated unsigned long will be bits 32-63 of the u64, so swap these two
* ulong values within the u64.
*/
static void swap_ulong_in_u64(u64 *bits, unsigned int nr)
{
#if (BITS_PER_LONG == 32) && defined(__BIG_ENDIAN)
unsigned int i;
for (i = 0; i < nr; i++)
bits[i] = (bits[i] >> 32) | ((u64)(u32)bits[i] << 32);
#endif
}
/**
* bpf_iter_bits_new() - Initialize a new bits iterator for a given memory area
* @it: The new bpf_iter_bits to be created
@ -2904,6 +2927,8 @@ bpf_iter_bits_new(struct bpf_iter_bits *it, const u64 *unsafe_ptr__ign, u32 nr_w
if (err)
return -EFAULT;
swap_ulong_in_u64(&kit->bits_copy, nr_words);
kit->nr_bits = nr_bits;
return 0;
}
@ -2922,6 +2947,8 @@ bpf_iter_bits_new(struct bpf_iter_bits *it, const u64 *unsafe_ptr__ign, u32 nr_w
return err;
}
swap_ulong_in_u64(kit->bits, nr_words);
kit->nr_bits = nr_bits;
return 0;
}
@ -2939,7 +2966,7 @@ __bpf_kfunc int *bpf_iter_bits_next(struct bpf_iter_bits *it)
{
struct bpf_iter_bits_kern *kit = (void *)it;
int bit = kit->bit, nr_bits = kit->nr_bits;
const unsigned long *bits;
const void *bits;
if (!nr_bits || bit >= nr_bits)
return NULL;