[PATCH v2] arm: ptrace: Fix mask for thumb breakpoint hook

[PATCH v2] arm: ptrace: Fix mask for thumb breakpoint hook

[Fredrik Strupe]

call_undef_hook() in traps.c applies the same instr_mask for both 16-bit
and 32-bit thumb instructions. If instr_mask then is only 16 bits wide
(0xffff as opposed to 0xffffffff), the first half-word of 32-bit thumb
instructions will be masked out. This makes the function match 32-bit
thumb instructions where the second half-word is equal to instr_val,
regardless of the first half-word.

The result in this case is that all undefined 32-bit thumb instructions
with the second half-word equal to de01 (udf #1) work as breakpoints
and will raise a SIGTRAP instead of a SIGILL, instead of just the one
intended 16-bit instruction. An example of such an instruction is
eaa0b650, which is unallocated according to Arm ARM and should raise a
SIGILL, but instead raises a SIGTRAP.

This patch fixes the issue by setting all the bits in instr_mask, which
will still match the intended 16-bit thumb instruction (where the
upper half is always 0), but not any 32-bit thumb instructions.

Signed-off-by: Fredrik Strupe <fredrik@strupe.net>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Russell King <linux@armlinux.org.uk>
---
 arch/arm/kernel/ptrace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index b606cded90cd..4cc6a7eff635 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -219,8 +219,8 @@ static struct undef_hook arm_break_hook = {
 };
 
 static struct undef_hook thumb_break_hook = {
-	.instr_mask	= 0xffff,
-	.instr_val	= 0xde01,
+	.instr_mask	= 0xffffffff,
+	.instr_val	= 0x0000de01,
 	.cpsr_mask	= PSR_T_BIT,
 	.cpsr_val	= PSR_T_BIT,
 	.fn		= break_trap,
-- 
2.20.1


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[PING] [PATCH v2] arm: ptrace: Fix mask for thumb breakpoint hook

[Fredrik Strupe]

call_undef_hook() in traps.c applies the same instr_mask for both 16-bit
and 32-bit thumb instructions. If instr_mask then is only 16 bits wide
(0xffff as opposed to 0xffffffff), the first half-word of 32-bit thumb
instructions will be masked out. This makes the function match 32-bit
thumb instructions where the second half-word is equal to instr_val,
regardless of the first half-word.

The result in this case is that all undefined 32-bit thumb instructions
with the second half-word equal to de01 (udf #1) work as breakpoints
and will raise a SIGTRAP instead of a SIGILL, instead of just the one
intended 16-bit instruction. An example of such an instruction is
eaa0b650, which is unallocated according to Arm ARM and should raise a
SIGILL, but instead raises a SIGTRAP.

This patch fixes the issue by setting all the bits in instr_mask, which
will still match the intended 16-bit thumb instruction (where the
upper half is always 0), but not any 32-bit thumb instructions.

Signed-off-by: Fredrik Strupe <fredrik@strupe.net>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Russell King <linux@armlinux.org.uk>
---
 arch/arm/kernel/ptrace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index b606cded90cd..4cc6a7eff635 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -219,8 +219,8 @@ static struct undef_hook arm_break_hook = {
 };
 
 static struct undef_hook thumb_break_hook = {
-	.instr_mask	= 0xffff,
-	.instr_val	= 0xde01,
+	.instr_mask	= 0xffffffff,
+	.instr_val	= 0x0000de01,
 	.cpsr_mask	= PSR_T_BIT,
 	.cpsr_val	= PSR_T_BIT,
 	.fn		= break_trap,
-- 
2.20.1


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Re: [PING] [PATCH v2] arm: ptrace: Fix mask for thumb breakpoint hook

[Russell King - ARM Linux admin]

On Mon, May 18, 2020 at 03:12:06PM +0200, Fredrik Strupe wrote:
> call_undef_hook() in traps.c applies the same instr_mask for both 16-bit
> and 32-bit thumb instructions. If instr_mask then is only 16 bits wide
> (0xffff as opposed to 0xffffffff), the first half-word of 32-bit thumb
> instructions will be masked out. This makes the function match 32-bit
> thumb instructions where the second half-word is equal to instr_val,
> regardless of the first half-word.
> 
> The result in this case is that all undefined 32-bit thumb instructions
> with the second half-word equal to de01 (udf #1) work as breakpoints
> and will raise a SIGTRAP instead of a SIGILL, instead of just the one
> intended 16-bit instruction. An example of such an instruction is
> eaa0b650, which is unallocated according to Arm ARM and should raise a
> SIGILL, but instead raises a SIGTRAP.

How can 0xeaa0b650 match 0xde01 when masked with 0xffff ?

-- 
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Re: [PING] [PATCH v2] arm: ptrace: Fix mask for thumb breakpoint hook

[Fredrik Strupe]

On 18.05.2020 16:18, Russell King - ARM Linux admin wrote:
> On Mon, May 18, 2020 at 03:12:06PM +0200, Fredrik Strupe wrote:
>> call_undef_hook() in traps.c applies the same instr_mask for both 16-bit
>> and 32-bit thumb instructions. If instr_mask then is only 16 bits wide
>> (0xffff as opposed to 0xffffffff), the first half-word of 32-bit thumb
>> instructions will be masked out. This makes the function match 32-bit
>> thumb instructions where the second half-word is equal to instr_val,
>> regardless of the first half-word.
>>
>> The result in this case is that all undefined 32-bit thumb instructions
>> with the second half-word equal to de01 (udf #1) work as breakpoints
>> and will raise a SIGTRAP instead of a SIGILL, instead of just the one
>> intended 16-bit instruction. An example of such an instruction is
>> eaa0b650, which is unallocated according to Arm ARM and should raise a
>> SIGILL, but instead raises a SIGTRAP.
>
> How can 0xeaa0b650 match 0xde01 when masked with 0xffff ?
>

Sorry, that is a typo; it should say 0xeaa0de01.

For reference, this is similar to the problem with SETEND emulation that
was fixed in commit fc2266011acc in the mainline kernel
(or as discussed here: https://lkml.org/lkml/2020/4/8/274).

Fredrik


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Re: [PING] [PATCH v2] arm: ptrace: Fix mask for thumb breakpoint hook

[Russell King - ARM Linux admin]

On Mon, May 18, 2020 at 05:02:27PM +0200, Fredrik Strupe wrote:
> On 18.05.2020 16:18, Russell King - ARM Linux admin wrote:
> > On Mon, May 18, 2020 at 03:12:06PM +0200, Fredrik Strupe wrote:
> >> call_undef_hook() in traps.c applies the same instr_mask for both 16-bit
> >> and 32-bit thumb instructions. If instr_mask then is only 16 bits wide
> >> (0xffff as opposed to 0xffffffff), the first half-word of 32-bit thumb
> >> instructions will be masked out. This makes the function match 32-bit
> >> thumb instructions where the second half-word is equal to instr_val,
> >> regardless of the first half-word.
> >>
> >> The result in this case is that all undefined 32-bit thumb instructions
> >> with the second half-word equal to de01 (udf #1) work as breakpoints
> >> and will raise a SIGTRAP instead of a SIGILL, instead of just the one
> >> intended 16-bit instruction. An example of such an instruction is
> >> eaa0b650, which is unallocated according to Arm ARM and should raise a
> >> SIGILL, but instead raises a SIGTRAP.
> >
> > How can 0xeaa0b650 match 0xde01 when masked with 0xffff ?
> >
> 
> Sorry, that is a typo; it should say 0xeaa0de01.
> 
> For reference, this is similar to the problem with SETEND emulation that
> was fixed in commit fc2266011acc in the mainline kernel
> (or as discussed here: https://lkml.org/lkml/2020/4/8/274).

Thanks for the clarification.  Please update the patch description and
put it in the patch system, thanks.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 10.2Mbps down 587kbps up

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