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SubjectRe: [RFC PATCH v1 44/50] arm64: ptr auth: Use get_random_u64 instead of _bytes
Hi George,

On Tue, Dec 10, 2019 at 07:15:55AM -0500, George Spelvin wrote:
> Since these are authentication keys, stored in the kernel as long
> as they're important, get_random_u64 is fine. In particular,
> get_random_bytes has significant per-call overhead, so five
> separate calls is painful.

As I am unaware, how does the cost of get_random_bytes() compare to the
cost of get_random_u64()?

> This ended up being a more extensive change, since the previous
> code was unrolled and 10 calls to get_random_u64() seems excessive.
> So the code was rearranged to have smaller object size.

It's not really "unrolled", but rather "not a loop", so I'd prefer to
not artifially make it look like one.

Could you please quantify the size difference when going from
get_random_bytes() to get_random_u64(), if that is excessive enough to
warrant changing the structure of the code? Otherwise please leave the
structure as-is as given it is much easier to reason about -- suggestion
below on how to do that neatly.

> Currently fields[i] = { 1 << i, 16 * i } for all i could be computed
> rather than looked up, but the table seemed more future-proof.
>
> For ptrauth_keys_switch(), the MSR instructions must be unrolled and
> are much faster than get_random, so although a similar flags-based
> interface is possible, it's probably not worth it.
>
> Signed-off-by: George Spelvin <lkml@sdf.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: linux-arm-kernel@lists.infradead.org
> ---
> arch/arm64/include/asm/pointer_auth.h | 20 +++++----
> arch/arm64/kernel/pointer_auth.c | 62 +++++++++++++++------------
> 2 files changed, 46 insertions(+), 36 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
> index 7a24bad1a58b8..b7ef71362a3ae 100644
> --- a/arch/arm64/include/asm/pointer_auth.h
> +++ b/arch/arm64/include/asm/pointer_auth.h
> @@ -30,17 +30,19 @@ struct ptrauth_keys {
> struct ptrauth_key apga;
> };
>
> +static inline unsigned long ptrauth_keys_supported(void)
> +{
> + return (system_supports_address_auth() ?
> + PR_PAC_APIAKEY | PR_PAC_APIBKEY |
> + PR_PAC_APDAKEY | PR_PAC_APDBKEY : 0) |
> + (system_supports_generic_auth() ? PR_PAC_APGAKEY : 0);
> +}
> +
> +void ptrauth_keys_generate(struct ptrauth_keys *keys, unsigned long flags);
> +
> static inline void ptrauth_keys_init(struct ptrauth_keys *keys)
> {
> - if (system_supports_address_auth()) {
> - get_random_bytes(&keys->apia, sizeof(keys->apia));
> - get_random_bytes(&keys->apib, sizeof(keys->apib));
> - get_random_bytes(&keys->apda, sizeof(keys->apda));
> - get_random_bytes(&keys->apdb, sizeof(keys->apdb));
> - }
> -
> - if (system_supports_generic_auth())
> - get_random_bytes(&keys->apga, sizeof(keys->apga));
> + ptrauth_keys_generate(keys, ptrauth_keys_supported());
> }
>
> #define __ptrauth_key_install(k, v) \
> diff --git a/arch/arm64/kernel/pointer_auth.c b/arch/arm64/kernel/pointer_auth.c
> index c507b584259d0..1604ed246128c 100644
> --- a/arch/arm64/kernel/pointer_auth.c
> +++ b/arch/arm64/kernel/pointer_auth.c
> @@ -7,40 +7,48 @@
> #include <asm/cpufeature.h>
> #include <asm/pointer_auth.h>
>
> +/*
> + * Generating crypto-quality random numbers is expensive enough that
> + * there's no point unrolling this.
> + */
> +void ptrauth_keys_generate(struct ptrauth_keys *keys, unsigned long flags)
> +{
> + size_t i;
> + static const struct {
> + /*
> + * 8 bits is enough for now. Compiler will complain
> + * if/when we need more.
> + */
> + unsigned char flag, offset;
> + } fields[] = {
> + { PR_PAC_APIAKEY, offsetof(struct ptrauth_keys, apia) },
> + { PR_PAC_APIBKEY, offsetof(struct ptrauth_keys, apib) },
> + { PR_PAC_APDAKEY, offsetof(struct ptrauth_keys, apda) },
> + { PR_PAC_APDBKEY, offsetof(struct ptrauth_keys, apdb) },
> + { PR_PAC_APGAKEY, offsetof(struct ptrauth_keys, apga) }
> + };
> +
> + for (i = 0; i < ARRAY_SIZE(fields); i++) {
> + if (flags & fields[i].flag) {
> + struct ptrauth_key *k = (void *)keys + fields[i].offset;
> + k->lo = get_random_u64();
> + k->hi = get_random_u64();
> + }
> + }
> +}

If we must use get_random_u64() in place of get_random_bytes(), please
add a per-key helper and keep the structure largely as-is:

// Note: we can drop inline if there is a real size issue
static inline void __ptrauth_key_init(struct ptrauth_key *key)
{
key->lo = get_random_u64();
key->hi = get_random_u64();
}

static inline void ptrauth_keys_init(struct ptrauth_keys *keys)
{
if (system_supports_address_auth()) {
__ptrauth_key_init(&keys->apia);
__ptrauth_key_init(&keys->apib);
__ptrauth_key_init(&keys->apda);
__ptrauth_key_init(&keys->apdb);
}

if (system_supports_generic_auth())
__ptrauth_key_init(&keys->apga);
ptrauth_keys_generate(keys, ptrauth_keys_supported());
}

> +
> int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg)
> {
> + unsigned long supported = ptrauth_keys_supported();
> struct ptrauth_keys *keys = &tsk->thread.keys_user;
> - unsigned long addr_key_mask = PR_PAC_APIAKEY | PR_PAC_APIBKEY |
> - PR_PAC_APDAKEY | PR_PAC_APDBKEY;
> - unsigned long key_mask = addr_key_mask | PR_PAC_APGAKEY;
>
> - if (!system_supports_address_auth() && !system_supports_generic_auth())
> + if (!supported || arg & ~supported)
> return -EINVAL;
>
> - if (!arg) {
> - ptrauth_keys_init(keys);
> - ptrauth_keys_switch(keys);
> - return 0;
> - }
> -
> - if (arg & ~key_mask)
> - return -EINVAL;
> -
> - if (((arg & addr_key_mask) && !system_supports_address_auth()) ||
> - ((arg & PR_PAC_APGAKEY) && !system_supports_generic_auth()))
> - return -EINVAL;
> -
> - if (arg & PR_PAC_APIAKEY)
> - get_random_bytes(&keys->apia, sizeof(keys->apia));
> - if (arg & PR_PAC_APIBKEY)
> - get_random_bytes(&keys->apib, sizeof(keys->apib));
> - if (arg & PR_PAC_APDAKEY)
> - get_random_bytes(&keys->apda, sizeof(keys->apda));
> - if (arg & PR_PAC_APDBKEY)
> - get_random_bytes(&keys->apdb, sizeof(keys->apdb));
> - if (arg & PR_PAC_APGAKEY)
> - get_random_bytes(&keys->apga, sizeof(keys->apga));
> + if (!arg)
> + arg = supported;

... and similarly this should only need to change to use
__ptrauth_key_init(), as above.

Thanks,
Mark.

>
> + ptrauth_keys_generate(keys, arg);
> ptrauth_keys_switch(keys);
>
> return 0;
> --
> 2.26.0
>

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