[PATCH kernel] powerpc/kuap: Restore AMR after replaying soft interrupts

Wed Feb 3 19:45:03 AEDT 2021

Hello,

On Tue, Feb 02, 2021 at 08:15:41PM +1100, Alexey Kardashevskiy wrote:
> Since de78a9c "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.
> 
> 890274c "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.
> 
> This saves/restores AMR when replaying interrupts.
> 
> This adds a check if AMR was not blocked when before replaying interrupts.
> 
> Found by syzkaller. The call stack for the bug is:
> 
> copy_from_user_nofault+0xf8/0x250
> perf_callchain_user_64+0x3d8/0x8d0
> perf_callchain_user+0x38/0x50
> get_perf_callchain+0x28c/0x300
> perf_callchain+0xb0/0x130
> perf_prepare_sample+0x364/0xbf0
> perf_event_output_forward+0xe0/0x280
> __perf_event_overflow+0xa4/0x240
> perf_swevent_hrtimer+0x1d4/0x1f0
> __hrtimer_run_queues+0x328/0x900
> hrtimer_interrupt+0x128/0x350
> timer_interrupt+0x180/0x600
> replay_soft_interrupts+0x21c/0x4f0
> arch_local_irq_restore+0x94/0x150
> lock_is_held_type+0x140/0x200
> ___might_sleep+0x220/0x330
> __might_fault+0x88/0x120
> do_strncpy_from_user+0x108/0x2b0
> strncpy_from_user+0x1d0/0x2a0
> getname_flags+0x88/0x2c0
> do_sys_openat2+0x2d4/0x5f0
> do_sys_open+0xcc/0x140
> system_call_exception+0x160/0x240
> system_call_common+0xf0/0x27c
> 
Can we get a Fixes tag?

Thanks

Michal
> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
> Reviewed-by: Nicholas Piggin <npiggin at gmail.com>
> ---
> Changes:
> v3:
> * do not block/unblock if AMR was blocked
> * reverted move of AMR_KUAP_***
> * added pr_warn
> 
> v2:
> * fixed compile on hash
> * moved get/set to arch_local_irq_restore
> * block KUAP before replaying
> 
> ---
> 
> This is an example:
> 
> ------------[ cut here ]------------
> 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 __do_page_fau
> 
> Modules linked in:
> 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 [c00000000009ece8] __do_page_fault+0xb38/0xde0
> LR [c00000000009ece4] __do_page_fault+0xb34/0xde0
> Call Trace:
> [c00000000dc637f0] [c00000000009ece4] __do_page_fault+0xb34/0xde0 (unreliable)
> [c00000000dc638a0] [c00000000000c968] handle_page_fault+0x10/0x2c
> --- interrupt: 300 at strncpy_from_user+0x290/0x440
>     LR = strncpy_from_user+0x284/0x440
> [c00000000dc63ba0] [c000000000c3dcb0] strncpy_from_user+0x2f0/0x440 (unreliable)
> [c00000000dc63c30] [c00000000068b888] getname_flags+0x88/0x2c0
> [c00000000dc63c90] [c000000000662a44] do_sys_openat2+0x2d4/0x5f0
> [c00000000dc63d30] [c00000000066560c] do_sys_open+0xcc/0x140
> [c00000000dc63dc0] [c000000000045e10] system_call_exception+0x160/0x240
> [c00000000dc63e20] [c00000000000da60] system_call_common+0xf0/0x27c
> Instruction dump:
> 409c0048 3fe2ff5b 3bfff128 fac10060 fae10068 482f7a85 60000000 3c62ff5b
> 7fe4fb78 3863f250 4815bbd9 60000000 <0fe00000> 3c62ff5b 3863f2b8 4815c8b5
> irq event stamp: 254
> hardirqs last  enabled at (253): [<c000000000019550>] arch_local_irq_restore+0xa0/0x150
> hardirqs last disabled at (254): [<c000000000008a10>] data_access_common_virt+0x1b0/0x1d0
> softirqs last  enabled at (0): [<c0000000001f6d5c>] copy_process+0x78c/0x2120
> softirqs last disabled at (0): [<0000000000000000>] 0x0
> ---[ end trace ba98aec5151f3aeb ]---
> ---
>  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 cc7a6271b6b4..592abc798826 100644
> --- a/arch/powerpc/kernel/irq.c
> +++ b/arch/powerpc/kernel/irq.c
> @@ -269,6 +269,23 @@ void replay_soft_interrupts(void)
>  	}
>  }
>  
> +#if defined(CONFIG_PPC_BOOK3S_64) && defined(CONFIG_PPC_KUAP)
> +static inline void replay_soft_interrupts_irqrestore(void)
> +{
> +	unsigned long kuap_state = get_kuap();
> +
> +	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;
> @@ -278,6 +295,14 @@ notrace void arch_local_irq_restore(unsigned long mask)
>  	if (mask)
>  		return;
>  
> +	/*
> +	 * It fires if anything calls local_irq_enable or restore when
> +	 * KUAP is enabled, and the code handles that just fine by saving
> +	 * and re-locking AMR but we would like to remove those calls,
> +	 * hence the warning.
> +	 */
> +	kuap_check_amr();
> +
>  	/*
>  	 * From this point onward, we can take interrupts, preempt,
>  	 * etc... unless we got hard-disabled. We check if an event
> @@ -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();
> -- 
> 2.17.1
>