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Subject[PATCH 5.10 363/663] powerpc/kuap: Restore AMR after replaying soft interrupts
Date
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();
--
2.27.0


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