The patch titled Subject: prctl: generalize PR_SET_MDWE support check to be per-arch has been added to the -mm mm-hotfixes-unstable branch. Its filename is prctl-generalize-pr_set_mdwe-support-check-to-be-per-arch.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Zev Weiss zev@bewilderbeest.net Subject: prctl: generalize PR_SET_MDWE support check to be per-arch Date: Mon, 26 Feb 2024 17:35:41 -0800

Patch series "ARM: prctl: Reject PR_SET_MDWE where not supported".

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.

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.

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

This patch (of 2):

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.

Link: https://lkml.kernel.org/r/20240227013546.15769-4-zev@bewilderbeest.net Link: https://lkml.kernel.org/r/20240227013546.15769-5-zev@bewilderbeest.net Signed-off-by: Zev Weiss zev@bewilderbeest.net Acked-by: Helge Deller deller@gmx.de [parisc] Cc: Borislav Petkov bp@alien8.de Cc: David Hildenbrand david@redhat.com Cc: Florent Revest revest@chromium.org Cc: "James E.J. Bottomley" James.Bottomley@HansenPartnership.com Cc: Josh Triplett josh@joshtriplett.org Cc: Kees Cook keescook@chromium.org Cc: Miguel Ojeda ojeda@kernel.org Cc: Mike Rapoport (IBM) rppt@kernel.org Cc: Oleg Nesterov oleg@redhat.com Cc: Ondrej Mosnacek omosnace@redhat.com Cc: Rick Edgecombe rick.p.edgecombe@intel.com Cc: Russell King (Oracle) linux@armlinux.org.uk Cc: Sam James sam@gentoo.org Cc: Stefan Roesch shr@devkernel.io Cc: Yang Shi yang@os.amperecomputing.com Cc: Yin Fengwei fengwei.yin@intel.com Cc: stable@vger.kernel.org [6.3+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

arch/parisc/include/asm/mman.h | 14 ++++++++++++++ include/linux/mman.h | 8 ++++++++ kernel/sys.c | 7 +++++-- 3 files changed, 27 insertions(+), 2 deletions(-)

--- /dev/null +++ a/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__ */ --- a/include/linux/mman.h~prctl-generalize-pr_set_mdwe-support-check-to-be-per-arch +++ a/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. * --- a/kernel/sys.c~prctl-generalize-pr_set_mdwe-support-check-to-be-per-arch +++ a/kernel/sys.c @@ -2408,8 +2408,11 @@ static inline int prctl_set_mdwe(unsigne 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(); _

Patches currently in -mm which might be from zev@bewilderbeest.net are

prctl-generalize-pr_set_mdwe-support-check-to-be-per-arch.patch arm-prctl-reject-pr_set_mdwe-on-pre-armv6.patch