[PATCH v8 2/7] powerpc/kprobes: Mark newly allocated probes as RO

[Naveen N. Rao]

Russell Currey wrote:
> With CONFIG_STRICT_KERNEL_RWX=y and CONFIG_KPROBES=y, there will be one
> W+X page at boot by default.  This can be tested with
> CONFIG_PPC_PTDUMP=y and CONFIG_PPC_DEBUG_WX=y set, and checking the
> kernel log during boot.
> 
> powerpc doesn't implement its own alloc() for kprobes like other
> architectures do, but we couldn't immediately mark RO anyway since we do
> a memcpy to the page we allocate later.  After that, nothing should be
> allowed to modify the page, and write permissions are removed well
> before the kprobe is armed.
> 
> The memcpy() would fail if >1 probes were allocated, so use
> patch_instruction() instead which is safe for RO.
> 
> Reviewed-by: Daniel Axtens <dja at axtens.net>
> Signed-off-by: Russell Currey <ruscur at russell.cc>
> Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
> ---
>  arch/powerpc/kernel/kprobes.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> index 81efb605113e..fa4502b4de35 100644
> --- a/arch/powerpc/kernel/kprobes.c
> +++ b/arch/powerpc/kernel/kprobes.c
> @@ -24,6 +24,8 @@
>  #include <asm/sstep.h>
>  #include <asm/sections.h>
>  #include <linux/uaccess.h>
> +#include <linux/set_memory.h>
> +#include <linux/vmalloc.h>
>  
>  DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
>  DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
> @@ -102,6 +104,16 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
>  	return addr;
>  }
>  
> +void *alloc_insn_page(void)
> +{
> +	void *page = vmalloc_exec(PAGE_SIZE);
> +
> +	if (page)
> +		set_memory_ro((unsigned long)page, 1);
> +
> +	return page;
> +}
> +

This crashes for me with KPROBES_SANITY_TEST during the kretprobe test.  
It seems to be handling the kretprobe itself properly, but seems to 
crash on the return path. I haven't yet been able to work out what's 
going wrong.

[    0.517880] Kprobe smoke test: started
[    0.626680] Oops: Exception in kernel mode, sig: 4 [#1]
[    0.626708] LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA PowerNV
[    0.626735] Modules linked in:
[    0.626760] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.6.0-06592-g2e64694b9137 #51
[    0.626795] NIP:  c008000000020000 LR: c00000000021ce34 CTR: c00000000021c5f8
[    0.626829] REGS: c0000000fd3e3860 TRAP: 0e40   Not tainted  (5.6.0-06592-g2e64694b9137)
[    0.626862] MSR:  9000000002089433 <SF,HV,VEC,EE,ME,SE,IR,DR,RI,LE>  CR: 28000284  XER: 00040000
[    0.626922] CFAR: c00000000000ef20 IRQMASK: 0 
[    0.626922] GPR00: c000000000052250 c0000000fd3e3af0 c000000002330200 000000002db8ad86 
[    0.626922] GPR04: 0000000000000000 c00000000006ba3c 0000000000000800 c0000000fd3a0000 
[    0.626922] GPR08: 0000000000000000 ffffffffaaaaaaab c0000000fd3a0000 0000000040000000 
[    0.626922] GPR12: c00000000021c5f0 c000000002520000 c000000000011790 0000000000000000 
[    0.626922] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
[    0.626922] GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
[    0.626922] GPR24: c0000000020034bc c0000000012068b8 c000000002062e50 c0000000fd2319a0 
[    0.626922] GPR28: c000000000f5ebb0 0000000000000000 c0000000021bc278 c000000002458540 
[    0.627234] NIP [c008000000020000] 0xc008000000020000
[    0.627264] LR [c00000000021ce34] init_test_probes+0x424/0x560
[    0.627291] Call Trace:
[    0.627313] [c0000000fd3e3af0] [c00000000021ce34] init_test_probes+0x424/0x560 (unreliable)
[    0.627356] [c0000000fd3e3b90] [c00000000202de2c] init_kprobes+0x1a8/0x1c8
[    0.627392] [c0000000fd3e3c00] [c000000000011140] do_one_initcall+0x60/0x2b0
[    0.627432] [c0000000fd3e3cd0] [c000000002004674] kernel_init_freeable+0x2e0/0x3a0
[    0.627471] [c0000000fd3e3db0] [c0000000000117ac] kernel_init+0x24/0x178
[    0.627510] [c0000000fd3e3e20] [c00000000000c7a8] ret_from_kernel_thread+0x5c/0x74
[    0.627543] Instruction dump:
[    0.627562] XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX 
[    0.627607] XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX <00000000> 00000000 00000000 00000000 
[    0.627660] ---[ end trace 964ab92781f5d99d ]---
[    0.629607] 


- Naveen