[PATCH] powerpc: Don't clobber fr0/vs0 during fp|altivec register save

Timothy Pearson tpearson at raptorengineering.com
Sun Nov 19 10:45:03 AEDT 2023

During floating point and vector save to thread data fr0/vs0 are clobbered
by the FPSCR/VSCR store routine.  This leads to userspace register corruption
and application data corruption / crash under the following rare condition:

 * A userspace thread is executing with VSX/FP mode enabled
 * The userspace thread is making active use of fr0 and/or vs0
 * An IPI is taken in kernel mode, forcing the userspace thread to reschedule
 * The userspace thread is interrupted by the IPI before accessing data it
   previously stored in fr0/vs0
 * The thread being switched in by the IPI has a pending signal

If these exact criteria are met, then the following sequence happens:

 * The existing thread FP storage is still valid before the IPI, due to a
   prior call to save_fpu() or store_fp_state().  Note that the current
   fr0/vs0 registers have been clobbered, so the FP/VSX state in registers
   is now invalid pending a call to restore_fp()/restore_altivec().
 * IPI -- FP/VSX register state remains invalid
 * interrupt_exit_user_prepare_main() calls do_notify_resume(),
   due to the pending signal
 * do_notify_resume() eventually calls save_fpu() via giveup_fpu(), which
   merrily reads and saves the invalid FP/VSX state to thread local storage.
 * interrupt_exit_user_prepare_main() calls restore_math(), writing the invalid
   FP/VSX state back to registers.
 * Execution is released to userspace, and the application crashes or corrupts

Without the pending signal, do_notify_resume() is never called, therefore the
invalid register state does't matter as it is overwritten nearly immeediately
by interrupt_exit_user_prepare_main() calling restore_math() before return
to userspace.

The combination of MariaDB and io_uring is especially good at triggering data
corruption using the above sequence, see MariaDB bug MDEV-30728.

Restore fr0/vs0 after FPSCR/VSCR store has completed for both the fp and
altivec register save paths.

Tested under QEMU in kvm mode, running on a Talos II workstation with dual

Tested-by: Timothy Pearson <tpearson at raptorengineering.com>
Signed-off-by: Timothy Pearson <tpearson at raptorengineering.com>
 arch/powerpc/kernel/fpu.S    | 13 +++++++++++++
 arch/powerpc/kernel/vector.S |  4 ++++
 2 files changed, 17 insertions(+)

diff --git a/arch/powerpc/kernel/fpu.S b/arch/powerpc/kernel/fpu.S
index 6a9acfb690c9..2f8f3f93cbb6 100644
--- a/arch/powerpc/kernel/fpu.S
+++ b/arch/powerpc/kernel/fpu.S
@@ -23,6 +23,15 @@
 #include <asm/feature-fixups.h>
 #ifdef CONFIG_VSX
+#define __REST_1FPVSR(n,c,base)						\
+	b	2f;							\
+	REST_FPR(n,base);						\
+	b	3f;							\
+2:	REST_VSR(n,c,base);						\
 #define __REST_32FPVSRS(n,c,base)					\
 	b	2f;							\
@@ -41,9 +50,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_VSX);					\
 2:	SAVE_32VSRS(n,c,base);						\
+#define __REST_1FPVSR(n,b,base)		REST_FPR(n, base)
 #define __REST_32FPVSRS(n,b,base)	REST_32FPRS(n, base)
 #define __SAVE_32FPVSRS(n,b,base)	SAVE_32FPRS(n, base)
+#define REST_1FPVSR(n,c,base)   __REST_1FPVSR(n,__REG_##c,__REG_##base)
 #define REST_32FPVSRS(n,c,base) __REST_32FPVSRS(n,__REG_##c,__REG_##base)
 #define SAVE_32FPVSRS(n,c,base) __SAVE_32FPVSRS(n,__REG_##c,__REG_##base)
@@ -67,6 +78,7 @@ _GLOBAL(store_fp_state)
 	SAVE_32FPVSRS(0, R4, R3)
 	mffs	fr0
 	stfd	fr0,FPSTATE_FPSCR(r3)
+	REST_1FPVSR(0, R4, R3)
@@ -138,4 +150,5 @@ _GLOBAL(save_fpu)
 2:	SAVE_32FPVSRS(0, R4, R6)
 	mffs	fr0
 	stfd	fr0,FPSTATE_FPSCR(r6)
+	REST_1FPVSR(0, R4, R6)
diff --git a/arch/powerpc/kernel/vector.S b/arch/powerpc/kernel/vector.S
index 4094e4c4c77a..8c63b05b421e 100644
--- a/arch/powerpc/kernel/vector.S
+++ b/arch/powerpc/kernel/vector.S
@@ -33,6 +33,8 @@ _GLOBAL(store_vr_state)
 	mfvscr	v0
 	stvx	v0, r4, r3
+	li	r4, 0
+	lvx	v0, r4, r3
@@ -109,6 +111,8 @@ _GLOBAL(save_altivec)
 	mfvscr	v0
 	stvx	v0,r4,r7
+	li	r4,0
+	lvx	v0,r4,r7
 #ifdef CONFIG_VSX

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