Re: [PATCH 2/5] mm/mseal: move madvise() logic to mm/madvise.c

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Lorenzo Stoakes <lorenzo.stoakes@oracle.com> [250714 09:08]:
> The madvise() logic is inexplicably performed in mm/mseal.c - this ought to
> be located in mm/madvise.c.
> 
> Additionally can_modify_vma_madv() is inconsistently named and, in
> combination with is_ro_anon(), is very confusing logic.
> 
> Put a static function in mm/madvise.c instead - can_madvise_modify() - that
> spells out exactly what's happening. Also explicitly check for an anon VMA.
> 
> Additionally add commentary to explain what's going on.
> 
> No functional change intended.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
>  mm/madvise.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>  mm/mseal.c   | 49 -----------------------------------------
>  mm/vma.h     |  7 ------
>  3 files changed, 61 insertions(+), 57 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 9de9b7c797c6..75757ba418a8 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -19,6 +19,7 @@
>  #include <linux/sched.h>
>  #include <linux/sched/mm.h>
>  #include <linux/mm_inline.h>
> +#include <linux/mmu_context.h>
>  #include <linux/string.h>
>  #include <linux/uio.h>
>  #include <linux/ksm.h>
> @@ -1256,6 +1257,65 @@ static long madvise_guard_remove(struct madvise_behavior *madv_behavior)
>  			       &guard_remove_walk_ops, NULL);
>  }
> 
> +#ifdef CONFIG_64BIT
> +/* Does the madvise operation result in discarding of mapped data? */
> +static bool is_discard(int behavior)
> +{
> +	switch (behavior) {
> +	case MADV_FREE:
> +	case MADV_DONTNEED:
> +	case MADV_DONTNEED_LOCKED:
> +	case MADV_REMOVE:
> +	case MADV_DONTFORK:
> +	case MADV_WIPEONFORK:
> +	case MADV_GUARD_INSTALL:
> +		return true;
> +	}
> +
> +	return false;
> +}
> +
> +/*
> + * We are restricted from madvise()'ing mseal()'d VMAs only in very particular
> + * circumstances - discarding of data from read-only anonymous SEALED mappings.
> + *
> + * This is because users cannot trivally discard data from these VMAs, and may
> + * only do so via an appropriate madvise() call.
> + */
> +static bool can_madvise_modify(struct madvise_behavior *madv_behavior)
> +{
> +	struct vm_area_struct *vma = madv_behavior->vma;
> +
> +	/* If the operation won't discard, we're good. */
> +	if (!is_discard(madv_behavior->behavior))
> +		return true;
> +
> +	/* If the VMA isn't sealed we're also good. */
> +	if (can_modify_vma(vma))
> +		return true;
> +
> +	/* File-backed mappings don't lose data on discard. */
> +	if (!vma_is_anonymous(vma))
> +		return true;
> +
> +	/*
> +	 * If the VMA is writable and the architecture permits write access,
> +	 * then discarding is fine.
> +	 */
> +	if ((vma->vm_flags & VM_WRITE) &&
> +	    arch_vma_access_permitted(vma, /* write= */ true,
> +			/* execute= */ false, /* foreign= */ false))
> +		return true;
> +
> +	return false;
> +}
> +#else
> +static bool can_madvise_modify(struct madvise_behavior *madv_behavior)
> +{
> +	return true;
> +}
> +#endif
> +
>  /*
>   * Apply an madvise behavior to a region of a vma.  madvise_update_vma
>   * will handle splitting a vm area into separate areas, each area with its own
> @@ -1269,7 +1329,7 @@ static int madvise_vma_behavior(struct madvise_behavior *madv_behavior)
>  	struct madvise_behavior_range *range = &madv_behavior->range;
>  	int error;
> 
> -	if (unlikely(!can_modify_vma_madv(madv_behavior->vma, behavior)))
> +	if (unlikely(!can_madvise_modify(madv_behavior)))
>  		return -EPERM;
> 
>  	switch (behavior) {
> diff --git a/mm/mseal.c b/mm/mseal.c
> index c27197ac04e8..1308e88ab184 100644
> --- a/mm/mseal.c
> +++ b/mm/mseal.c
> @@ -11,7 +11,6 @@
>  #include <linux/mman.h>
>  #include <linux/mm.h>
>  #include <linux/mm_inline.h>
> -#include <linux/mmu_context.h>
>  #include <linux/syscalls.h>
>  #include <linux/sched.h>
>  #include "internal.h"
> @@ -21,54 +20,6 @@ static inline void set_vma_sealed(struct vm_area_struct *vma)
>  	vm_flags_set(vma, VM_SEALED);
>  }
> 
> -static bool is_madv_discard(int behavior)
> -{
> -	switch (behavior) {
> -	case MADV_FREE:
> -	case MADV_DONTNEED:
> -	case MADV_DONTNEED_LOCKED:
> -	case MADV_REMOVE:
> -	case MADV_DONTFORK:
> -	case MADV_WIPEONFORK:
> -	case MADV_GUARD_INSTALL:
> -		return true;
> -	}
> -
> -	return false;
> -}
> -
> -static bool is_ro_anon(struct vm_area_struct *vma)
> -{
> -	/* check anonymous mapping. */
> -	if (vma->vm_file || vma->vm_flags & VM_SHARED)
> -		return false;
> -
> -	/*
> -	 * check for non-writable:
> -	 * PROT=RO or PKRU is not writeable.
> -	 */
> -	if (!(vma->vm_flags & VM_WRITE) ||
> -		!arch_vma_access_permitted(vma, true, false, false))
> -		return true;
> -
> -	return false;
> -}
> -
> -/*
> - * Check if a vma is allowed to be modified by madvise.
> - */
> -bool can_modify_vma_madv(struct vm_area_struct *vma, int behavior)
> -{
> -	if (!is_madv_discard(behavior))
> -		return true;
> -
> -	if (unlikely(!can_modify_vma(vma) && is_ro_anon(vma)))
> -		return false;
> -
> -	/* Allow by default. */
> -	return true;
> -}
> -
>  static int mseal_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		struct vm_area_struct **prev, unsigned long start,
>  		unsigned long end, vm_flags_t newflags)
> diff --git a/mm/vma.h b/mm/vma.h
> index cf6e3a6371b6..6515045ba342 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -578,8 +578,6 @@ static inline bool can_modify_vma(struct vm_area_struct *vma)
>  	return true;
>  }
> 
> -bool can_modify_vma_madv(struct vm_area_struct *vma, int behavior);
> -
>  #else
> 
>  static inline bool can_modify_vma(struct vm_area_struct *vma)
> @@ -587,11 +585,6 @@ static inline bool can_modify_vma(struct vm_area_struct *vma)
>  	return true;
>  }
> 
> -static inline bool can_modify_vma_madv(struct vm_area_struct *vma, int behavior)
> -{
> -	return true;
> -}
> -
>  #endif
> 
>  #if defined(CONFIG_STACK_GROWSUP)
> --
> 2.50.1