From: Alexey Kardashevskiy aik@ozlabs.ru
[ Upstream commit 60a707d0c99aff4eadb7fd334c5fd21df386723e ]
Since de78a9c42a79 ("powerpc: Add a framework for Kernel Userspace Access Protection"), user access helpers call user_{read|write}_access_{begin|end} when user space access is allowed.
Commit 890274c2dc4c ("powerpc/64s: Implement KUAP for Radix MMU") made the mentioned helpers program a AMR special register to allow such access for a short period of time, most of the time AMR is expected to block user memory access by the kernel.
Since the code accesses the user space memory, unsafe_get_user() calls might_fault() which calls arch_local_irq_restore() if either CONFIG_PROVE_LOCKING or CONFIG_DEBUG_ATOMIC_SLEEP is enabled. arch_local_irq_restore() then attempts to replay pending soft interrupts as KUAP regions have hardware interrupts enabled.
If a pending interrupt happens to do user access (performance interrupts do that), it enables access for a short period of time so after returning from the replay, the user access state remains blocked and if a user page fault happens - "Bug: Read fault blocked by AMR!" appears and SIGSEGV is sent.
An example trace: Bug: Read fault blocked by AMR! WARNING: CPU: 0 PID: 1603 at /home/aik/p/kernel/arch/powerpc/include/asm/book3s/64/kup-radix.h:145 CPU: 0 PID: 1603 Comm: amr Not tainted 5.10.0-rc6_v5.10-rc6_a+fstn1 #24 NIP: c00000000009ece8 LR: c00000000009ece4 CTR: 0000000000000000 REGS: c00000000dc63560 TRAP: 0700 Not tainted (5.10.0-rc6_v5.10-rc6_a+fstn1) MSR: 8000000000021033 <SF,ME,IR,DR,RI,LE> CR: 28002888 XER: 20040000 CFAR: c0000000001fa928 IRQMASK: 1 GPR00: c00000000009ece4 c00000000dc637f0 c000000002397600 000000000000001f GPR04: c0000000020eb318 0000000000000000 c00000000dc63494 0000000000000027 GPR08: c00000007fe4de68 c00000000dfe9180 0000000000000000 0000000000000001 GPR12: 0000000000002000 c0000000030a0000 0000000000000000 0000000000000000 GPR16: 0000000000000000 0000000000000000 0000000000000000 bfffffffffffffff GPR20: 0000000000000000 c0000000134a4020 c0000000019c2218 0000000000000fe0 GPR24: 0000000000000000 0000000000000000 c00000000d106200 0000000040000000 GPR28: 0000000000000000 0000000000000300 c00000000dc63910 c000000001946730 NIP __do_page_fault+0xb38/0xde0 LR __do_page_fault+0xb34/0xde0 Call Trace: __do_page_fault+0xb34/0xde0 (unreliable) handle_page_fault+0x10/0x2c --- interrupt: 300 at strncpy_from_user+0x290/0x440 LR = strncpy_from_user+0x284/0x440 strncpy_from_user+0x2f0/0x440 (unreliable) getname_flags+0x88/0x2c0 do_sys_openat2+0x2d4/0x5f0 do_sys_open+0xcc/0x140 system_call_exception+0x160/0x240 system_call_common+0xf0/0x27c
To fix it save/restore the AMR when replaying interrupts, and also add a check if AMR was not blocked prior to replaying interrupts.
Originally found by syzkaller.
Fixes: 890274c2dc4c ("powerpc/64s: Implement KUAP for Radix MMU") Signed-off-by: Alexey Kardashevskiy aik@ozlabs.ru Reviewed-by: Nicholas Piggin npiggin@gmail.com [mpe: Use normal commit citation format and add full oops log to change log, move kuap_check_amr() into the restore routine to avoid warnings about unreconciled IRQ state] Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20210202091541.36499-1-aik@ozlabs.ru Signed-off-by: Sasha Levin sashal@kernel.org --- arch/powerpc/kernel/irq.c | 27 ++++++++++++++++++++++++++- 1 file changed, 26 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c index cc7a6271b6b4e..e8a548447dd68 100644 --- a/arch/powerpc/kernel/irq.c +++ b/arch/powerpc/kernel/irq.c @@ -269,6 +269,31 @@ again: } }
+#if defined(CONFIG_PPC_BOOK3S_64) && defined(CONFIG_PPC_KUAP) +static inline void replay_soft_interrupts_irqrestore(void) +{ + unsigned long kuap_state = get_kuap(); + + /* + * Check if anything calls local_irq_enable/restore() when KUAP is + * disabled (user access enabled). We handle that case here by saving + * and re-locking AMR but we shouldn't get here in the first place, + * hence the warning. + */ + kuap_check_amr(); + + if (kuap_state != AMR_KUAP_BLOCKED) + set_kuap(AMR_KUAP_BLOCKED); + + replay_soft_interrupts(); + + if (kuap_state != AMR_KUAP_BLOCKED) + set_kuap(kuap_state); +} +#else +#define replay_soft_interrupts_irqrestore() replay_soft_interrupts() +#endif + notrace void arch_local_irq_restore(unsigned long mask) { unsigned char irq_happened; @@ -332,7 +357,7 @@ notrace void arch_local_irq_restore(unsigned long mask) irq_soft_mask_set(IRQS_ALL_DISABLED); trace_hardirqs_off();
- replay_soft_interrupts(); + replay_soft_interrupts_irqrestore(); local_paca->irq_happened = 0;
trace_hardirqs_on();