* [PATCH] powerpc/security: Use a mutex for interrupt exit code patching
@ 2021-10-27 7:24 Russell Currey
2021-10-28 3:03 ` Nicholas Piggin
2021-11-02 10:12 ` Michael Ellerman
0 siblings, 2 replies; 3+ messages in thread
From: Russell Currey @ 2021-10-27 7:24 UTC (permalink / raw)
To: linuxppc-dev; +Cc: npiggin, Russell Currey
The mitigation-patching.sh script in the powerpc selftests toggles
all mitigations on and off simultaneously, revealing that rfi_flush
and stf_barrier cannot safely operate at the same time due to races
in updating the static key.
On some systems, the static key code throws a warning and the kernel
remains functional. On others, the kernel will hang or crash.
Fix this by slapping on a mutex.
Fixes: 13799748b957 ("powerpc/64: use interrupt restart table to speed up return from interrupt")
Signed-off-by: Russell Currey <ruscur@russell.cc>
---
arch/powerpc/lib/feature-fixups.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/arch/powerpc/lib/feature-fixups.c b/arch/powerpc/lib/feature-fixups.c
index cda17bee5afe..9956277fbd88 100644
--- a/arch/powerpc/lib/feature-fixups.c
+++ b/arch/powerpc/lib/feature-fixups.c
@@ -228,6 +228,7 @@ static void do_stf_exit_barrier_fixups(enum stf_barrier_type types)
static bool stf_exit_reentrant = false;
static bool rfi_exit_reentrant = false;
+static DEFINE_MUTEX(exit_flush_lock);
static int __do_stf_barrier_fixups(void *data)
{
@@ -253,6 +254,9 @@ void do_stf_barrier_fixups(enum stf_barrier_type types)
* low level interrupt exit code before patching. After the patching,
* if allowed, then flip the branch to allow fast exits.
*/
+
+ // Prevent static key update races with do_rfi_flush_fixups()
+ mutex_lock(&exit_flush_lock);
static_branch_enable(&interrupt_exit_not_reentrant);
stop_machine(__do_stf_barrier_fixups, &types, NULL);
@@ -264,6 +268,7 @@ void do_stf_barrier_fixups(enum stf_barrier_type types)
if (stf_exit_reentrant && rfi_exit_reentrant)
static_branch_disable(&interrupt_exit_not_reentrant);
+ mutex_unlock(&exit_flush_lock);
}
void do_uaccess_flush_fixups(enum l1d_flush_type types)
@@ -486,6 +491,9 @@ void do_rfi_flush_fixups(enum l1d_flush_type types)
* without stop_machine, so this could be achieved with a broadcast
* IPI instead, but this matches the stf sequence.
*/
+
+ // Prevent static key update races with do_stf_barrier_fixups()
+ mutex_lock(&exit_flush_lock);
static_branch_enable(&interrupt_exit_not_reentrant);
stop_machine(__do_rfi_flush_fixups, &types, NULL);
@@ -497,6 +505,7 @@ void do_rfi_flush_fixups(enum l1d_flush_type types)
if (stf_exit_reentrant && rfi_exit_reentrant)
static_branch_disable(&interrupt_exit_not_reentrant);
+ mutex_unlock(&exit_flush_lock);
}
void do_barrier_nospec_fixups_range(bool enable, void *fixup_start, void *fixup_end)
--
2.33.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] powerpc/security: Use a mutex for interrupt exit code patching
2021-10-27 7:24 [PATCH] powerpc/security: Use a mutex for interrupt exit code patching Russell Currey
@ 2021-10-28 3:03 ` Nicholas Piggin
2021-11-02 10:12 ` Michael Ellerman
1 sibling, 0 replies; 3+ messages in thread
From: Nicholas Piggin @ 2021-10-28 3:03 UTC (permalink / raw)
To: linuxppc-dev, Russell Currey
Excerpts from Russell Currey's message of October 27, 2021 5:24 pm:
> The mitigation-patching.sh script in the powerpc selftests toggles
> all mitigations on and off simultaneously, revealing that rfi_flush
> and stf_barrier cannot safely operate at the same time due to races
> in updating the static key.
>
> On some systems, the static key code throws a warning and the kernel
> remains functional. On others, the kernel will hang or crash.
>
> Fix this by slapping on a mutex.
>
> Fixes: 13799748b957 ("powerpc/64: use interrupt restart table to speed up return from interrupt")
> Signed-off-by: Russell Currey <ruscur@russell.cc>
Doh,
Acked-by: Nicholas Piggin <npiggin@gmail.com>
> ---
> arch/powerpc/lib/feature-fixups.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/arch/powerpc/lib/feature-fixups.c b/arch/powerpc/lib/feature-fixups.c
> index cda17bee5afe..9956277fbd88 100644
> --- a/arch/powerpc/lib/feature-fixups.c
> +++ b/arch/powerpc/lib/feature-fixups.c
> @@ -228,6 +228,7 @@ static void do_stf_exit_barrier_fixups(enum stf_barrier_type types)
>
> static bool stf_exit_reentrant = false;
> static bool rfi_exit_reentrant = false;
> +static DEFINE_MUTEX(exit_flush_lock);
>
> static int __do_stf_barrier_fixups(void *data)
> {
> @@ -253,6 +254,9 @@ void do_stf_barrier_fixups(enum stf_barrier_type types)
> * low level interrupt exit code before patching. After the patching,
> * if allowed, then flip the branch to allow fast exits.
> */
> +
> + // Prevent static key update races with do_rfi_flush_fixups()
> + mutex_lock(&exit_flush_lock);
> static_branch_enable(&interrupt_exit_not_reentrant);
>
> stop_machine(__do_stf_barrier_fixups, &types, NULL);
> @@ -264,6 +268,7 @@ void do_stf_barrier_fixups(enum stf_barrier_type types)
>
> if (stf_exit_reentrant && rfi_exit_reentrant)
> static_branch_disable(&interrupt_exit_not_reentrant);
> + mutex_unlock(&exit_flush_lock);
> }
>
> void do_uaccess_flush_fixups(enum l1d_flush_type types)
> @@ -486,6 +491,9 @@ void do_rfi_flush_fixups(enum l1d_flush_type types)
> * without stop_machine, so this could be achieved with a broadcast
> * IPI instead, but this matches the stf sequence.
> */
> +
> + // Prevent static key update races with do_stf_barrier_fixups()
> + mutex_lock(&exit_flush_lock);
> static_branch_enable(&interrupt_exit_not_reentrant);
>
> stop_machine(__do_rfi_flush_fixups, &types, NULL);
> @@ -497,6 +505,7 @@ void do_rfi_flush_fixups(enum l1d_flush_type types)
>
> if (stf_exit_reentrant && rfi_exit_reentrant)
> static_branch_disable(&interrupt_exit_not_reentrant);
> + mutex_unlock(&exit_flush_lock);
> }
>
> void do_barrier_nospec_fixups_range(bool enable, void *fixup_start, void *fixup_end)
> --
> 2.33.1
>
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] powerpc/security: Use a mutex for interrupt exit code patching
2021-10-27 7:24 [PATCH] powerpc/security: Use a mutex for interrupt exit code patching Russell Currey
2021-10-28 3:03 ` Nicholas Piggin
@ 2021-11-02 10:12 ` Michael Ellerman
1 sibling, 0 replies; 3+ messages in thread
From: Michael Ellerman @ 2021-11-02 10:12 UTC (permalink / raw)
To: Russell Currey, linuxppc-dev; +Cc: npiggin
On Wed, 27 Oct 2021 17:24:10 +1000, Russell Currey wrote:
> The mitigation-patching.sh script in the powerpc selftests toggles
> all mitigations on and off simultaneously, revealing that rfi_flush
> and stf_barrier cannot safely operate at the same time due to races
> in updating the static key.
>
> On some systems, the static key code throws a warning and the kernel
> remains functional. On others, the kernel will hang or crash.
>
> [...]
Applied to powerpc/next.
[1/1] powerpc/security: Use a mutex for interrupt exit code patching
https://git.kernel.org/powerpc/c/3c12b4df8d5e026345a19886ae375b3ebc33c0b6
cheers
^ permalink raw reply [flat|nested] 3+ messages in thread
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2021-10-27 7:24 [PATCH] powerpc/security: Use a mutex for interrupt exit code patching Russell Currey
2021-10-28 3:03 ` Nicholas Piggin
2021-11-02 10:12 ` Michael Ellerman
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