The patch below does not apply to the 5.15-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 9d71165d3934e607070c4e48458c0cf161b1baea Mon Sep 17 00:00:00 2001 From: Nicholas Piggin npiggin@gmail.com Date: Fri, 11 Mar 2022 12:47:33 +1000 Subject: [PATCH] powerpc/tm: Fix more userspace r13 corruption

Commit cf13435b730a ("powerpc/tm: Fix userspace r13 corruption") fixes a problem in treclaim where a SLB miss can occur on the thread_struct->ckpt_regs while SCRATCH0 is live with the saved user r13 value, clobbering it with the kernel r13 and ultimately resulting in kernel r13 being stored in ckpt_regs.

There is an equivalent problem in trechkpt where the user r13 value is loaded into r13 from chkpt_regs to be recheckpointed, but a SLB miss could occur on ckpt_regs accesses after that, which will result in r13 being clobbered with a kernel value and that will get recheckpointed and then restored to user registers.

The same memory page is accessed right before this critical window where a SLB miss could cause corruption, so hitting the bug requires the SLB entry be removed within a small window of instructions, which is possible if a SLB related MCE hits there. PAPR also permits the hypervisor to discard this SLB entry (because slb_shadow->persistent is only set to SLB_NUM_BOLTED) although it's not known whether any implementations would do this (KVM does not). So this is an extremely unlikely bug, only found by inspection.

Fix this by also storing user r13 in a temporary location on the kernel stack and don't change the r13 register from kernel r13 until the RI=0 critical section that does not fault.

The SCRATCH0 change is not strictly part of the fix, it's only used in the RI=0 section so it does not have the same problem as the previous SCRATCH0 bug.

Fixes: 98ae22e15b43 ("powerpc: Add helper functions for transactional memory context switching") Cc: stable@vger.kernel.org # v3.9+ Signed-off-by: Nicholas Piggin npiggin@gmail.com Acked-by: Michael Neuling mikey@neuling.org Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20220311024733.48926-1-npiggin@gmail.com

diff --git a/arch/powerpc/kernel/tm.S b/arch/powerpc/kernel/tm.S index 3beecc32940b..5a0f023a26e9 100644 --- a/arch/powerpc/kernel/tm.S +++ b/arch/powerpc/kernel/tm.S @@ -443,7 +443,8 @@ restore_gprs:

REST_GPR(0, r7) /* GPR0 */ REST_GPRS(2, 4, r7) /* GPR2-4 */ - REST_GPRS(8, 31, r7) /* GPR8-31 */ + REST_GPRS(8, 12, r7) /* GPR8-12 */ + REST_GPRS(14, 31, r7) /* GPR14-31 */

/* Load up PPR and DSCR here so we don't run with user values for long */ mtspr SPRN_DSCR, r5 @@ -479,18 +480,24 @@ restore_gprs: REST_GPR(6, r7)

/* - * Store r1 and r5 on the stack so that we can access them after we - * clear MSR RI. + * Store user r1 and r5 and r13 on the stack (in the unused save + * areas / compiler reserved areas), so that we can access them after + * we clear MSR RI. */

REST_GPR(5, r7) std r5, -8(r1) - ld r5, GPR1(r7) + ld r5, GPR13(r7) std r5, -16(r1) + ld r5, GPR1(r7) + std r5, -24(r1)

REST_GPR(7, r7)

- /* Clear MSR RI since we are about to use SCRATCH0. EE is already off */ + /* Stash the stack pointer away for use after recheckpoint */ + std r1, PACAR1(r13) + + /* Clear MSR RI since we are about to clobber r13. EE is already off */ li r5, 0 mtmsrd r5, 1

@@ -501,9 +508,9 @@ restore_gprs: * until we turn MSR RI back on. */

- SET_SCRATCH0(r1) ld r5, -8(r1) - ld r1, -16(r1) + ld r13, -16(r1) + ld r1, -24(r1)

/* Commit register state as checkpointed state: */ TRECHKPT @@ -519,9 +526,9 @@ restore_gprs: */

GET_PACA(r13) - GET_SCRATCH0(r1) + ld r1, PACAR1(r13)

- /* R1 is restored, so we are recoverable again. EE is still off */ + /* R13, R1 is restored, so we are recoverable again. EE is still off */ li r4, MSR_RI mtmsrd r4, 1