[PATCH 5.14 839/849] powerpc/security: Use a mutex for interrupt exit code patching

Greg Kroah-Hartman
2021-11-16T01:12:54Z

From: Russell Currey <ruscur@russell.cc>

commit 3c12b4df8d5e026345a19886ae375b3ebc33c0b6 upstream.

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")
Cc: stable@vger.kernel.org # v5.14+
Signed-off-by: Russell Currey <ruscur@russell.cc>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211027072410.40950-1-ruscur@russell.cc
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/powerpc/lib/feature-fixups.c | 11 +++++++++++
1 file changed, 11 insertions(+)

--- 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(e

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_barr
* 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 +268,8 @@ void do_stf_barrier_fixups(enum stf_barr

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 +492,9 @@ void do_rfi_flush_fixups(enum l1d_flush_
* 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 +506,8 @@ void do_rfi_flush_fixups(enum l1d_flush_

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)