Re: [PATCH hotfix 6.12 v4 4/5] mm: refactor arch_calc_vm_flag_bits() and arm64 MTE handling

[Vlastimil Babka <vbabka@suse.cz>]

On 10/29/24 19:11, Lorenzo Stoakes 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 somewhere else.
> 
> We resolve this by setting VM_MTE_ALLOWED much earlier in do_mmap(), via
> the arch_calc_vm_flag_bits() call.
> 
> This is an appropriate place to do this as we already check for the
> MAP_ANONYMOUS case here, and the shmem file case is simply a variant of the
> same idea - we permit RAM-backed memory.
> 
> This requires a modification to the arch_calc_vm_flag_bits() 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 parisc.
> 
> So this patch performs this adjustment and removes the unnecessary
> assignment of VM_MTE_ALLOWED in shmem_mmap().
> 
> Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> Reported-by: Jann Horn <jannh@google.com>
> Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails")
> Cc: stable <stable@kernel.org>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

> --- a/arch/arm64/include/asm/mman.h
> +++ b/arch/arm64/include/asm/mman.h
> @@ -6,6 +6,8 @@
> 
>  #ifndef BUILD_VDSO
>  #include <linux/compiler.h>
> +#include <linux/fs.h>
> +#include <linux/shmem_fs.h>
>  #include <linux/types.h>
> 
>  static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
> @@ -31,19 +33,21 @@ static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
>  }
>  #define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey)
> 
> -static inline unsigned long arch_calc_vm_flag_bits(unsigned long flags)
> +static inline unsigned long arch_calc_vm_flag_bits(struct file *file,
> +						   unsigned long flags)
>  {
>  	/*
>  	 * Only allow MTE on anonymous mappings as these are guaranteed to be
>  	 * backed by tags-capable memory. The vm_flags may be overridden by a
>  	 * filesystem supporting MTE (RAM-based).

We should also eventually remove the last sentence or even replace it with
its negation, or somebody might try reintroducing the pattern that won't
work anymore (wasn't there such a hugetlbfs thing in -next?).

>  	 */
> -	if (system_supports_mte() && (flags & MAP_ANONYMOUS))
> +	if (system_supports_mte() &&
> +	    ((flags & MAP_ANONYMOUS) || shmem_file(file)))
>  		return VM_MTE_ALLOWED;
> 
>  	return 0;
>  }