[PATCH RESEND] arm64: add alignment fault hanling

[PATCH RESEND] arm64: add alignment fault hanling

[EunTaik Lee]

Userspace memory is mapped as below:
F2A7F000--F2A7FFFF Normal Memory
F2A80000--F2A80FFF Device nGnRnE

And that userspace application makes a system call
as below:

-009 |do_strncpy_from_user(inline)
-009 |strncpy_from_user()
-010 |getname_flags()
-011 |user_path_at_empty()
-012 |user_path_at()
-013 |SYSC_faccessat(inline)
-013 |sys_faccessat()
-014 |__sys_trace(asm)
--> |exception

The string spans from 0xF2A7FFC1 to 0xF2A7FFFB.

When do_strncpy_from_user() reads the last (unsigned long)
value, the alignement fault is triggered. The 8 byte
from 0xF2A7FFC1 spans to the next page that is mapped as
Device nGnRnE, which does not allow an unaligned access,
causes the abort.

The instruction which caused the alignment fault is registered
in the fixup table but the exception handler does not reach there.

This patch registers a alignment fault handler and fixes up the
pc if appropriate.

Signed-off-by: Eun Taik Lee 
---
arch/arm64/mm/fault.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 19211c4..8257d4f 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -371,6 +371,16 @@ static int __kprobes do_translation_fault(unsigned long addr,
return 0;
}

+static int __kprobes do_alignment_fault(unsigned long addr,
+   unsigned int esr,
+   struct pt_regs *regs)
+{
+ if (addr >= TASK_SIZE && fixup_exception(regs))
+ return 0;
+
+ return 1;
+}
+
/*
  * This abort handler always returns "fault".
  */
@@ -418,7 +428,7 @@ static struct fault_info {
{ do_bad, SIGBUS,  0, "synchronous parity error (translation table walk" },
{ do_bad, SIGBUS,  0, "synchronous parity error (translation table walk" },
{ do_bad, SIGBUS,  0, "unknown 32" },
- { do_bad, SIGBUS,  BUS_ADRALN, "alignment fault" },
+ { do_alignment_fault, SIGBUS,  BUS_ADRALN, "alignment fault" },
{ do_bad, SIGBUS,  0, "debug event" },
{ do_bad, SIGBUS,  0, "unknown 35" },
{ do_bad, SIGBUS,  0, "unknown 36" },
-- 
1.9.1

[PATCH RESEND] arm64: add alignment fault hanling

[Catalin Marinas]

On Mon, Feb 15, 2016 at 08:58:32AM +0000, EunTaik Lee wrote:
> Userspace memory is mapped as below:
> F2A7F000--F2A7FFFF Normal Memory
> F2A80000--F2A80FFF Device nGnRnE

How do you end up with Device nGnRnE in user space? I thought we should
have got some guard page.

> And that userspace application makes a system call
> as below:
> 
> -009 |do_strncpy_from_user(inline)
> -009 |strncpy_from_user()
> -010 |getname_flags()
> -011 |user_path_at_empty()
> -012 |user_path_at()
> -013 |SYSC_faccessat(inline)
> -013 |sys_faccessat()
> -014 |__sys_trace(asm)
> --> |exception
> 
> The string spans from 0xF2A7FFC1 to 0xF2A7FFFB.
> 
> When do_strncpy_from_user() reads the last (unsigned long)
> value, the alignement fault is triggered. The 8 byte
> from 0xF2A7FFC1 spans to the next page that is mapped as
> Device nGnRnE, which does not allow an unaligned access,
> causes the abort.

do_strncpy_from_user() relies on unsafe_get_user() not being able to
read 8 bytes. The problem now is that it doesn't get a page fault but an
alignment one is isn't handled.

> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 19211c4..8257d4f 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -371,6 +371,16 @@ static int __kprobes do_translation_fault(unsigned long addr,
> return 0;
> }
> 
> +static int __kprobes do_alignment_fault(unsigned long addr,
> +   unsigned int esr,
> +   struct pt_regs *regs)
> +{
> + if (addr >= TASK_SIZE && fixup_exception(regs))
> + return 0;
> +
> + return 1;
> +}

Why addr >= TASK_SIZE? addr here should be the fault address, not pc.

> +
> /*
>   * This abort handler always returns "fault".
>   */
> @@ -418,7 +428,7 @@ static struct fault_info {
> { do_bad, SIGBUS,  0, "synchronous parity error (translation table walk" },
> { do_bad, SIGBUS,  0, "synchronous parity error (translation table walk" },
> { do_bad, SIGBUS,  0, "unknown 32" },
> - { do_bad, SIGBUS,  BUS_ADRALN, "alignment fault" },
> + { do_alignment_fault, SIGBUS,  BUS_ADRALN, "alignment fault" },

The simplest would be to use do_bad_area() here without any additional
function.

-- 
Catalin