[PATCH RESEND 0/2] ARM: prctl: Reject PR_SET_MDWE where not supported

[PATCH RESEND 0/2] ARM: prctl: Reject PR_SET_MDWE where not supported

[Zev Weiss]

[Resending as per Russell's request...]

Hello,

I noticed after a recent kernel update that my ARM926 system started
segfaulting on any execve() after calling prctl(PR_SET_MDWE).  After
some investigation it appears that ARMv5 is incapable of providing the
appropriate protections for MDWE, since any readable memory is also
implicitly executable.

(Note that I'm not an expert in either ARM arch details or the mm
subsystem, so please bear with me if I've botched something in the
above analysis.)

The prctl_set_mdwe() function already had some special-case logic
added disabling it on PARISC (commit 793838138c15, "prctl: Disable
prctl(PR_SET_MDWE) on parisc"); this patch series (1) generalizes that
check to use an arch_*() function, and (2) adds a corresponding
override for ARM to disable MDWE on pre-ARMv6 CPUs.

With the series applied, prctl(PR_SET_MDWE) is rejected on ARMv5 and
subsequent execve() calls (as well as mmap(PROT_READ|PROT_WRITE)) can
succeed instead of unconditionally failing; on ARMv6 the prctl works
as it did previously.


Thanks,
Zev

[0] https://lore.kernel.org/all/2023112456-linked-nape-bf19@gregkh/

Zev Weiss (2):
  prctl: Generalize PR_SET_MDWE support check to be per-arch
  ARM: prctl: Reject PR_SET_MDWE on pre-ARMv6

 arch/arm/include/asm/mman.h    | 14 ++++++++++++++
 arch/parisc/include/asm/mman.h | 14 ++++++++++++++
 include/linux/mman.h           |  8 ++++++++
 kernel/sys.c                   |  7 +++++--
 4 files changed, 41 insertions(+), 2 deletions(-)
 create mode 100644 arch/arm/include/asm/mman.h
 create mode 100644 arch/parisc/include/asm/mman.h

-- 
2.43.2


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[PATCH RESEND 1/2] prctl: Generalize PR_SET_MDWE support check to be per-arch

[Zev Weiss]

There exist systems other than PARISC where MDWE may not be feasible
to support; rather than cluttering up the generic code with additional
arch-specific logic let's add a generic function for checking MDWE
support and allow each arch to override it as needed.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Cc: <stable@vger.kernel.org> # v6.3+
---
 arch/parisc/include/asm/mman.h | 14 ++++++++++++++
 include/linux/mman.h           |  8 ++++++++
 kernel/sys.c                   |  7 +++++--
 3 files changed, 27 insertions(+), 2 deletions(-)
 create mode 100644 arch/parisc/include/asm/mman.h

diff --git a/arch/parisc/include/asm/mman.h b/arch/parisc/include/asm/mman.h
new file mode 100644
index 000000000000..47c5a1991d10
--- /dev/null
+++ b/arch/parisc/include/asm/mman.h
@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_MMAN_H__
+#define __ASM_MMAN_H__
+
+#include <uapi/asm/mman.h>
+
+/* PARISC cannot allow mdwe as it needs writable stacks */
+static inline bool arch_memory_deny_write_exec_supported(void)
+{
+	return false;
+}
+#define arch_memory_deny_write_exec_supported arch_memory_deny_write_exec_supported
+
+#endif /* __ASM_MMAN_H__ */
diff --git a/include/linux/mman.h b/include/linux/mman.h
index dc7048824be8..bcb201ab7a41 100644
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -162,6 +162,14 @@ calc_vm_flag_bits(unsigned long flags)
 
 unsigned long vm_commit_limit(void);
 
+#ifndef arch_memory_deny_write_exec_supported
+static inline bool arch_memory_deny_write_exec_supported(void)
+{
+	return true;
+}
+#define arch_memory_deny_write_exec_supported arch_memory_deny_write_exec_supported
+#endif
+
 /*
  * Denies creating a writable executable mapping or gaining executable permissions.
  *
diff --git a/kernel/sys.c b/kernel/sys.c
index f8e543f1e38a..8bb106a56b3a 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2408,8 +2408,11 @@ static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
 	if (bits & PR_MDWE_NO_INHERIT && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
 		return -EINVAL;
 
-	/* PARISC cannot allow mdwe as it needs writable stacks */
-	if (IS_ENABLED(CONFIG_PARISC))
+	/*
+	 * EOPNOTSUPP might be more appropriate here in principle, but
+	 * existing userspace depends on EINVAL specifically.
+	 */
+	if (!arch_memory_deny_write_exec_supported())
 		return -EINVAL;
 
 	current_bits = get_current_mdwe();
-- 
2.43.2


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Re: [PATCH RESEND 1/2] prctl: Generalize PR_SET_MDWE support check to be per-arch

[Russell King (Oracle)]

On Mon, Feb 26, 2024 at 05:35:41PM -0800, Zev Weiss wrote:
> There exist systems other than PARISC where MDWE may not be feasible
> to support; rather than cluttering up the generic code with additional
> arch-specific logic let's add a generic function for checking MDWE
> support and allow each arch to override it as needed.
> 
> Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
> Cc: <stable@vger.kernel.org> # v6.3+

PA-RISC folk need to ack/review-by this patch. Alternatively, it needs
to be restructured to add the arch_memory_deny_write_exec_supported()
override without touching the PA-RISC code, which then makes the Arm
patch independent of the status of the PA-RISC patch. That will allow
the Arm issue to be solved even if an ack is not forthcoming for the
PA-RISC parts.

Alternatively, I wonder whether akpm would be willing to pick up this
patch set as-is.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!

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Re: [PATCH RESEND 1/2] prctl: Generalize PR_SET_MDWE support check to be per-arch

[Helge Deller]

On 2/27/24 11:24, Russell King (Oracle) wrote:
> On Mon, Feb 26, 2024 at 05:35:41PM -0800, Zev Weiss wrote:
>> There exist systems other than PARISC where MDWE may not be feasible
>> to support; rather than cluttering up the generic code with additional
>> arch-specific logic let's add a generic function for checking MDWE
>> support and allow each arch to override it as needed.
>>
>> Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
>> Cc: <stable@vger.kernel.org> # v6.3+
>
> PA-RISC folk need to ack/review-by this patch.

I'm fine with patch 1/2:
Acked-by: Helge Deller <deller@gmx.de> # parisc

> Alternatively, it needs
> to be restructured to add the arch_memory_deny_write_exec_supported()
> override without touching the PA-RISC code, which then makes the Arm
> patch independent of the status of the PA-RISC patch. That will allow
> the Arm issue to be solved even if an ack is not forthcoming for the
> PA-RISC parts.
>> Alternatively, I wonder whether akpm would be willing to pick up this
> patch set as-is.

I have no preference, but I think both patches should be pushed
together via arm tree or akpm.

Helge

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Re: [PATCH RESEND 1/2] prctl: Generalize PR_SET_MDWE support check to be per-arch

[Zev Weiss]

On Tue, Feb 27, 2024 at 11:53:59AM PST, Helge Deller wrote:
>On 2/27/24 11:24, Russell King (Oracle) wrote:
>>On Mon, Feb 26, 2024 at 05:35:41PM -0800, Zev Weiss wrote:
>>>There exist systems other than PARISC where MDWE may not be feasible
>>>to support; rather than cluttering up the generic code with additional
>>>arch-specific logic let's add a generic function for checking MDWE
>>>support and allow each arch to override it as needed.
>>>
>>>Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
>>>Cc: <stable@vger.kernel.org> # v6.3+
>>
>>PA-RISC folk need to ack/review-by this patch.
>
>I'm fine with patch 1/2:
>Acked-by: Helge Deller <deller@gmx.de> # parisc
>
>>Alternatively, it needs
>>to be restructured to add the arch_memory_deny_write_exec_supported()
>>override without touching the PA-RISC code, which then makes the Arm
>>patch independent of the status of the PA-RISC patch. That will allow
>>the Arm issue to be solved even if an ack is not forthcoming for the
>>PA-RISC parts.
>>>Alternatively, I wonder whether akpm would be willing to pick up this
>>patch set as-is.
>
>I have no preference, but I think both patches should be pushed
>together via arm tree or akpm.
>
>Helge

Ping...Russell, Andrew, any thoughts on how this could move forward?


Thanks,
Zev


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[PATCH RESEND 2/2] ARM: prctl: Reject PR_SET_MDWE on pre-ARMv6

[Zev Weiss]

On v5 and lower CPUs we can't provide MDWE protection, so ensure we
fail any attempt to enable it via prctl(PR_SET_MDWE).

Previously such an attempt would misleadingly succeed, leading to any
subsequent mmap(PROT_READ|PROT_WRITE) or execve() failing
unconditionally (the latter somewhat violently via
force_fatal_sig(SIGSEGV) due to READ_IMPLIES_EXEC).

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Cc: <stable@vger.kernel.org> # v6.3+
---
 arch/arm/include/asm/mman.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
 create mode 100644 arch/arm/include/asm/mman.h

diff --git a/arch/arm/include/asm/mman.h b/arch/arm/include/asm/mman.h
new file mode 100644
index 000000000000..2189e507c8e0
--- /dev/null
+++ b/arch/arm/include/asm/mman.h
@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_MMAN_H__
+#define __ASM_MMAN_H__
+
+#include <asm/system_info.h>
+#include <uapi/asm/mman.h>
+
+static inline bool arch_memory_deny_write_exec_supported(void)
+{
+	return cpu_architecture() >= CPU_ARCH_ARMv6;
+}
+#define arch_memory_deny_write_exec_supported arch_memory_deny_write_exec_supported
+
+#endif /* __ASM_MMAN_H__ */
-- 
2.43.2


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