Re: [PATCH -next] crypto: asymmetric_keys: verify_pefile: fix integer underflow in pefile_strip_sig_wrapper

[Herbert Xu]

On Mon, Nov 02, 2026 at 03:49:34PM +0800, Gu Bowen wrote:
> The pefile_strip_sig_wrapper() function validates the win_certificate
> wrapper that precedes the PKCS#7 signature in a PE binary.  It checks
> that the wrapper length does not exceed the certificate table size
> (``wrapper.length > ctx->sig_len``) but fails to verify that the
> wrapper length is at least ``sizeof(struct win_certificate)`` (8 bytes).
> 
> When a crafted PE image supplies a wrapper with ``length`` in the range
> [1, 7], the subsequent arithmetic::
> 
>     ctx->sig_len = wrapper.length;      /* e.g. 1 */
>     ctx->sig_offset += sizeof(wrapper);
>     ctx->sig_len -= sizeof(wrapper);    /* 1 - 8 = 0xFFFFFFF9 (unsigned) */
> 
> underflows the unsigned ``ctx->sig_len`` to a value near UINT_MAX.  The
> following ``sig_len < 4`` sanity check is bypassed because 0xFFFFFFF9 is
> greater than 4.
> 
> The underflowed ``sig_len`` is then passed to verify_pkcs7_signature()
> and onwards to asn1_ber_decoder().  The ASN.1 decoder's own
> ``datalen > 65535`` entry guard can be satisfied by constructing a
> PKCS#7 length field of exactly 65535 (e.g. ``30 82 ff fb``), allowing
> the decoder to proceed and attempt to read OID octets beyond the
> allocated vmalloc buffer, triggering a page fault in look_up_OID()::
> 
>   BUG: unable to handle page fault for address: ffffd1e986cb2000
>   #PF: supervisor read access in kernel mode
>   RIP: 0010:look_up_OID+0x14/0x100
>   Call Trace:
>    pkcs7_note_OID+0x33/0xa0
>    asn1_ber_decoder+0x363/0xbd0
>    pkcs7_parse_message+0xd9/0x1b0
>    verify_pkcs7_signature+0x24/0x80
>    verify_pefile_signature+0x7b/0xf0
>    kexec_kernel_verify_pe_sig+0x1b/0x40
>    kimage_file_alloc_init+0x1d8/0x3f0
>    __se_sys_kexec_file_load+0xc6/0x270
> 
> This is reachable through the kexec_file_load syscall on architectures
> that verify PE signatures (x86 bzImage with
> CONFIG_KEXEC_BZIMAGE_VERIFY_SIG, arm64 Image with
> CONFIG_KEXEC_IMAGE_VERIFY_SIG), requiring CAP_SYS_BOOT.
> 
> The oops occurs while the caller holds kexec_lock; if the faulting task
> is killed, the lock is never released, leaving all subsequent kexec and
> kdump operations permanently returning -EBUSY until reboot.
> 
> Add a lower-bound check on wrapper.length before the subtraction to
> prevent the underflow.
> 
> Fixes: 09dacbbda935 ("pefile: Strip the wrapper off of the cert data block")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Gu Bowen <gubowen5@xxxxxxxxxx>
> ---
>  crypto/asymmetric_keys/verify_pefile.c | 5 +++++
>  1 file changed, 5 insertions(+)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt