| Message ID | 202003311544.02VFiClP011630@sdf.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] arm64: ptr auth: Use get_random_u64 instead of _bytes | expand |
This seems to have been sent twice, so I'm replying to the latest copy. On Tue, Mar 31, 2020 at 03:44:12PM +0000, George Spelvin wrote: > get_random_bytes() is approximately 4x the cost of two > get_random_u64() calls, because the former implements > anti-backtracking. > > Because these are authentication keys, useless to an attacker > as soon as the kernel stops using them, there is no security > benefit from anti-backtracking. > > Signed-off-by: George Spelvin <lkml@sdf.org> > Cc: Mark Rutland <mark.rutland@arm.com> > Cc: Will Deacon <will@kernel.org> > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: linux-arm-kernel@lists.infradead.org Given my understanding from discussion on the prior version the conversion is sound, it doesn't matter that we expose the keys via ptrace, and the code looks nicer, so: Acked-by: Mark Rutland <mark.rutland@arm.com> I assume Will or Catalin will pick this up when they next pick up patches. Mark. > --- > v2: Took out all the clever bitmap-based stuff and made a simple > boring helper function to replace get_random_bytes(&key, 16). > > arch/arm64/include/asm/pointer_auth.h | 16 +++++++++++----- > arch/arm64/kernel/pointer_auth.c | 10 +++++----- > 2 files changed, 16 insertions(+), 10 deletions(-) > > diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h > index 7a24bad1a58b8..1a4590d05785e 100644 > --- a/arch/arm64/include/asm/pointer_auth.h > +++ b/arch/arm64/include/asm/pointer_auth.h > @@ -30,17 +30,23 @@ struct ptrauth_keys { > struct ptrauth_key apga; > }; > > +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()) { > - 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)); > + __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()) > - get_random_bytes(&keys->apga, sizeof(keys->apga)); > + __ptrauth_key_init(&keys->apga); > } > > #define __ptrauth_key_install(k, v) \ > diff --git a/arch/arm64/kernel/pointer_auth.c b/arch/arm64/kernel/pointer_auth.c > index c507b584259d0..05e2e3d174010 100644 > --- a/arch/arm64/kernel/pointer_auth.c > +++ b/arch/arm64/kernel/pointer_auth.c > @@ -31,15 +31,15 @@ int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg) > return -EINVAL; > > if (arg & PR_PAC_APIAKEY) > - get_random_bytes(&keys->apia, sizeof(keys->apia)); > + __ptrauth_key_init(&keys->apia); > if (arg & PR_PAC_APIBKEY) > - get_random_bytes(&keys->apib, sizeof(keys->apib)); > + __ptrauth_key_init(&keys->apib); > if (arg & PR_PAC_APDAKEY) > - get_random_bytes(&keys->apda, sizeof(keys->apda)); > + __ptrauth_key_init(&keys->apda); > if (arg & PR_PAC_APDBKEY) > - get_random_bytes(&keys->apdb, sizeof(keys->apdb)); > + __ptrauth_key_init(&keys->apdb); > if (arg & PR_PAC_APGAKEY) > - get_random_bytes(&keys->apga, sizeof(keys->apga)); > + __ptrauth_key_init(&keys->apga); > > ptrauth_keys_switch(keys); > > -- > 2.26.0
Hi George, On Tue, Mar 31, 2020 at 03:44:12PM +0000, George Spelvin wrote: > get_random_bytes() is approximately 4x the cost of two > get_random_u64() calls, because the former implements > anti-backtracking. > > Because these are authentication keys, useless to an attacker > as soon as the kernel stops using them, there is no security > benefit from anti-backtracking. > > Signed-off-by: George Spelvin <lkml@sdf.org> > Cc: Mark Rutland <mark.rutland@arm.com> > Cc: Will Deacon <will@kernel.org> > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: linux-arm-kernel@lists.infradead.org > --- > v2: Took out all the clever bitmap-based stuff and made a simple > boring helper function to replace get_random_bytes(&key, 16). > > arch/arm64/include/asm/pointer_auth.h | 16 +++++++++++----- > arch/arm64/kernel/pointer_auth.c | 10 +++++----- > 2 files changed, 16 insertions(+), 10 deletions(-) Please can you resend this against the arm64 for-next/ptr-auth branch [1]? I can't apply it as-is. Thanks, Will [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/ptr-auth
On Tue, Apr 28, 2020 at 01:58:12PM +0100, Will Deacon wrote: > On Tue, Mar 31, 2020 at 03:44:12PM +0000, George Spelvin wrote: > > get_random_bytes() is approximately 4x the cost of two > > get_random_u64() calls, because the former implements > > anti-backtracking. > > > > Because these are authentication keys, useless to an attacker > > as soon as the kernel stops using them, there is no security > > benefit from anti-backtracking. > > > > Signed-off-by: George Spelvin <lkml@sdf.org> > > Cc: Mark Rutland <mark.rutland@arm.com> > > Cc: Will Deacon <will@kernel.org> > > Cc: Catalin Marinas <catalin.marinas@arm.com> > > Cc: linux-arm-kernel@lists.infradead.org > > --- > > v2: Took out all the clever bitmap-based stuff and made a simple > > boring helper function to replace get_random_bytes(&key, 16). > > > > arch/arm64/include/asm/pointer_auth.h | 16 +++++++++++----- > > arch/arm64/kernel/pointer_auth.c | 10 +++++----- > > 2 files changed, 16 insertions(+), 10 deletions(-) > > Please can you resend this against the arm64 for-next/ptr-auth branch [1]? > I can't apply it as-is. Any update on this one? Cheers, Will > [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=for-next/ptr-auth
diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h index 7a24bad1a58b8..1a4590d05785e 100644 --- a/arch/arm64/include/asm/pointer_auth.h +++ b/arch/arm64/include/asm/pointer_auth.h @@ -30,17 +30,23 @@ struct ptrauth_keys { struct ptrauth_key apga; }; +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()) { - 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)); + __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()) - get_random_bytes(&keys->apga, sizeof(keys->apga)); + __ptrauth_key_init(&keys->apga); } #define __ptrauth_key_install(k, v) \ diff --git a/arch/arm64/kernel/pointer_auth.c b/arch/arm64/kernel/pointer_auth.c index c507b584259d0..05e2e3d174010 100644 --- a/arch/arm64/kernel/pointer_auth.c +++ b/arch/arm64/kernel/pointer_auth.c @@ -31,15 +31,15 @@ int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg) return -EINVAL; if (arg & PR_PAC_APIAKEY) - get_random_bytes(&keys->apia, sizeof(keys->apia)); + __ptrauth_key_init(&keys->apia); if (arg & PR_PAC_APIBKEY) - get_random_bytes(&keys->apib, sizeof(keys->apib)); + __ptrauth_key_init(&keys->apib); if (arg & PR_PAC_APDAKEY) - get_random_bytes(&keys->apda, sizeof(keys->apda)); + __ptrauth_key_init(&keys->apda); if (arg & PR_PAC_APDBKEY) - get_random_bytes(&keys->apdb, sizeof(keys->apdb)); + __ptrauth_key_init(&keys->apdb); if (arg & PR_PAC_APGAKEY) - get_random_bytes(&keys->apga, sizeof(keys->apga)); + __ptrauth_key_init(&keys->apga); ptrauth_keys_switch(keys);
get_random_bytes() is approximately 4x the cost of two get_random_u64() calls, because the former implements anti-backtracking. Because these are authentication keys, useless to an attacker as soon as the kernel stops using them, there is no security benefit from anti-backtracking. Signed-off-by: George Spelvin <lkml@sdf.org> Cc: Mark Rutland <mark.rutland@arm.com> Cc: Will Deacon <will@kernel.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: linux-arm-kernel@lists.infradead.org --- v2: Took out all the clever bitmap-based stuff and made a simple boring helper function to replace get_random_bytes(&key, 16). arch/arm64/include/asm/pointer_auth.h | 16 +++++++++++----- arch/arm64/kernel/pointer_auth.c | 10 +++++----- 2 files changed, 16 insertions(+), 10 deletions(-)