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Message-ID: <CAHbLzkqmDyOb9kwLKyKkThjT41UNCwb6p8XL6rEHVmvEbJEXZw@mail.gmail.com>
Date: Tue, 29 Oct 2024 08:43:04 -0700
From: Yang Shi <shy828301@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Peter Xu <peterx@...hat.com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Mark Brown <broonie@...nel.org>, 
	"David S . Miller" <davem@...emloft.net>, Andreas Larsson <andreas@...sler.com>
Subject: Re: [PATCH hotfix 6.12 v3 4/5] mm: refactor arch_validate_flags() and
 arm64 MTE handling

On Tue, Oct 29, 2024 at 6:04 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> Currently MTE is permitted in two circumstances (desiring to use MTE having
> been specified by the VM_MTE flag) - where MAP_ANONYMOUS is specified, as
> checked by arch_calc_vm_flag_bits() and actualised by setting the
> VM_MTE_ALLOWED flag, or if the file backing the mapping is shmem, in which
> case we set VM_MTE_ALLOWED in shmem_mmap() when the mmap hook is activated
> in mmap_region().
>
> The function that checks that, if VM_MTE is set, VM_MTE_ALLOWED is also set
> is the arm64 implementation of arch_validate_flags().
>
> Unfortunately, we intend to refactor mmap_region() to perform this check
> earlier, meaning that in the case of a shmem backing we will not have
> invoked shmem_mmap() yet, causing the mapping to fail spuriously.
>
> It is inappropriate to set this architecture-specific flag in general mm
> code anyway, so a sensible resolution of this issue is to instead move the
> check to arch_validate_flags() itself.
>
> This requires a modification to the arch_validate_flags() signature to pass
> in a pointer to the struct file associated with the mapping, however this
> is not too egregious as this is only used by two architectures anyway -
> arm64 and sparc.
>
> So this patch performs this adjustment and moves the check to
> arch_validate_flags() which resolves the issue.
>
> We'd ideally like to have eliminated the arch_calc_vm_flag_bits() case, but
> we risk inadvertently changing behaviour as we do not have mmap() flags
> available at the point of the arch_validate_flags() check and a MAP_ANON |
> MAP_HUGETLB case would be accepted for MTE currently, but a MAP_SHARED |
> MAP_HUGETLB would not.
>
> This is likely an oversight but we want to try to keep behaviour identical
> to before in this patch.
>
> So continue to check VM_MTE_ALLOWED which arch_calc_vm_flag_bits() sets if
> MAP_ANON.
>
> Reported-by: Jann Horn <jannh@...gle.com>
> Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails")
> Cc: stable <stable@...nel.org>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> Tested-by: Mark Brown <broonie@...nel.org>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
>  arch/arm64/include/asm/mman.h | 29 +++++++++++++++++++++++++----
>  arch/sparc/include/asm/mman.h |  5 +++--
>  include/linux/mman.h          |  2 +-
>  mm/mmap.c                     |  2 +-
>  mm/mprotect.c                 |  2 +-
>  mm/shmem.c                    |  3 ---
>  6 files changed, 31 insertions(+), 12 deletions(-)
>
> diff --git a/arch/arm64/include/asm/mman.h b/arch/arm64/include/asm/mman.h
> index 9e39217b4afb..94925a0482e3 100644
> --- a/arch/arm64/include/asm/mman.h
> +++ b/arch/arm64/include/asm/mman.h
> @@ -6,7 +6,9 @@
>
>  #ifndef BUILD_VDSO
>  #include <linux/compiler.h>
> +#include <linux/fs.h>
>  #include <linux/types.h>
> +#include <linux/shmem_fs.h>
>
>  static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
>         unsigned long pkey)
> @@ -60,15 +62,34 @@ static inline bool arch_validate_prot(unsigned long prot,
>  }
>  #define arch_validate_prot(prot, addr) arch_validate_prot(prot, addr)
>
> -static inline bool arch_validate_flags(unsigned long vm_flags)
> +static inline bool arch_validate_flags(struct file *file, unsigned long vm_flags)
>  {
>         if (!system_supports_mte())
>                 return true;
>
> -       /* only allow VM_MTE if VM_MTE_ALLOWED has been set previously */
> -       return !(vm_flags & VM_MTE) || (vm_flags & VM_MTE_ALLOWED);
> +       if (!(vm_flags & VM_MTE))
> +               return true;
> +
> +       /*
> +        * We explicitly permit MAP_ANONYMOUS and shmem backed mappings to use
> +        * MTE. We check for shmem explicitly here.
> +        *
> +        * The MAP_ANONYMOUS case is handled by arch_calc_vm_flag_bits() which
> +        * explicitly sets VM_MTE_ALLOWED which we check for here also.
> +        *
> +        * Ideally we'd perform both checks here but existing semantics support
> +        * hugetlb in MAP_ANONYMOUS mode but not a MAP_SHARED mapping, which is
> +        * likely unintended but we maintain identical behaviour for
> +        * consistency.
> +        */
> +       if (vm_flags & VM_MTE_ALLOWED)
> +               return true;
> +       if (shmem_file(file))
> +               return true;

Sorry for chiming in late. This looks problematic IIUC. The patch 5/5
moved arch_validate_flags() before call_mmap() as the commit log said.
But shmem_file() does this check:
mapping->a_ops == &shmem_aops

But mapping->a_ops is not initialized until shmem_mmap(), which is
called by call_mmap().

So shmem_file() should always return false here. Did I miss something?

> +
> +       return false;
>  }
> -#define arch_validate_flags(vm_flags) arch_validate_flags(vm_flags)
> +#define arch_validate_flags(file, vm_flags) arch_validate_flags(file, vm_flags)
>
>  #endif /* !BUILD_VDSO */
>
> diff --git a/arch/sparc/include/asm/mman.h b/arch/sparc/include/asm/mman.h
> index af9c10c83dc5..d426e1f7c2c1 100644
> --- a/arch/sparc/include/asm/mman.h
> +++ b/arch/sparc/include/asm/mman.h
> @@ -10,6 +10,7 @@ int sparc_mmap_check(unsigned long addr, unsigned long len);
>
>  #ifdef CONFIG_SPARC64
>  #include <asm/adi_64.h>
> +#include <linux/fs.h>
>
>  static inline void ipi_set_tstate_mcde(void *arg)
>  {
> @@ -54,11 +55,11 @@ static inline int sparc_validate_prot(unsigned long prot, unsigned long addr)
>         return 1;
>  }
>
> -#define arch_validate_flags(vm_flags) arch_validate_flags(vm_flags)
> +#define arch_validate_flags(file, vm_flags) arch_validate_flags(file, vm_flags)
>  /* arch_validate_flags() - Ensure combination of flags is valid for a
>   *     VMA.
>   */
> -static inline bool arch_validate_flags(unsigned long vm_flags)
> +static inline bool arch_validate_flags(struct file *file, unsigned long vm_flags)
>  {
>         /* If ADI is being enabled on this VMA, check for ADI
>          * capability on the platform and ensure VMA is suitable
> diff --git a/include/linux/mman.h b/include/linux/mman.h
> index 8ddca62d6460..82e6488026b7 100644
> --- a/include/linux/mman.h
> +++ b/include/linux/mman.h
> @@ -117,7 +117,7 @@ static inline bool arch_validate_prot(unsigned long prot, unsigned long addr)
>   *
>   * Returns true if the VM_* flags are valid.
>   */
> -static inline bool arch_validate_flags(unsigned long flags)
> +static inline bool arch_validate_flags(struct file *file, unsigned long flags)
>  {
>         return true;
>  }
> diff --git a/mm/mmap.c b/mm/mmap.c
> index ab71d4c3464c..40b5858ae875 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1511,7 +1511,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>         }
>
>         /* Allow architectures to sanity-check the vm_flags */
> -       if (!arch_validate_flags(vma->vm_flags)) {
> +       if (!arch_validate_flags(file, vma->vm_flags)) {
>                 error = -EINVAL;
>                 goto close_and_free_vma;
>         }
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 6f450af3252e..c6db98b893fc 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -816,7 +816,7 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
>                 }
>
>                 /* Allow architectures to sanity-check the new flags */
> -               if (!arch_validate_flags(newflags)) {
> +               if (!arch_validate_flags(vma->vm_file, newflags)) {
>                         error = -EINVAL;
>                         break;
>                 }
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 4ba1d00fabda..e87f5d6799a7 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2733,9 +2733,6 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
>         if (ret)
>                 return ret;
>
> -       /* arm64 - allow memory tagging on RAM-based files */
> -       vm_flags_set(vma, VM_MTE_ALLOWED);
> -
>         file_accessed(file);
>         /* This is anonymous shared memory if it is unlinked at the time of mmap */
>         if (inode->i_nlink)
> --
> 2.47.0
>

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