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libfido2: Address CHERI compatibility

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Cherry-picked from libfido2 upstream f20a735c0a6f:

iso7816: Avoid storing pointers in a packed structure

On CHERI, and thus Arm's experimental Morello prototype architecture,
pointers are represented as capabilities, which are unforgeable bounded
pointers, providing always-on fine-grained spatial memory safety. The
unforgeability is enforced through the use of tagged memory, with one
validity tag bit per capability-sized-and-aligned word in memory. This
means that storing a pointer to an unaligned location, which is not
guaranteed to work per the C standard, either traps or results in the
capability losing its tag (and thus never being dereferenceable again),
depending on how exactly the store is done (specifically, whether a
capability store or memcpy is used).

However, iso7816 itself does not need to be packed, and doing so likely
causes inefficiencies on existing architectures. The iso7816_header_t
member is packed, and the flexible payload array is a uint8_t (which by
definition has no padding bits and is exactly 8 bits in size and, since
CHAR_BITS must be at least 8, its existence implies that it has the same
representation as unsigned char, and that it has size and alignment 1)
so there will never be any padding inserted between header and payload
(but payload may overlap with padding at the end of the struct due to
how flexible arrays work, which means we need to be careful about our
calculations).

Co-authored-by: pedro martelletto <pedro@yubico.com>
(cherry picked from commit 224a95f124)
This commit is contained in:
Jessica Clarke 2021-10-02 16:51:38 +01:00 committed by Ed Maste
parent a32212fb59
commit 5d14c1d0f4
2 changed files with 4 additions and 5 deletions
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4
contrib/libfido2/src/iso7816.c
View File

@ -59,6 +59,6 @@ iso7816_ptr(const iso7816_apdu_t *apdu)
size_t
iso7816_len(const iso7816_apdu_t *apdu)
{
return apdu->alloc_len - sizeof(apdu->alloc_len) -
sizeof(apdu->payload_len) - sizeof(apdu->payload_ptr);
return apdu->alloc_len - offsetof(iso7816_apdu_t, header) -
(sizeof(iso7816_apdu_t) - offsetof(iso7816_apdu_t, payload));
}

5
contrib/libfido2/src/iso7816.h
View File

@ -27,14 +27,13 @@ struct iso7816_header {
uint8_t lc3;
})
PACKED_TYPE(iso7816_apdu_t,
struct iso7816_apdu {
typedef struct iso7816_apdu {
size_t alloc_len;
uint16_t payload_len;
uint8_t *payload_ptr;
iso7816_header_t header;
uint8_t payload[];
})
} iso7816_apdu_t;
const unsigned char *iso7816_ptr(const iso7816_apdu_t *);
int iso7816_add(iso7816_apdu_t *, const void *, size_t);