Commit d0b47a68 authored by Lorenzo Stoakes's avatar Lorenzo Stoakes Committed by Andrew Morton
Browse files

mm/mseal: update madvise() logic 

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.

Also add commentary to explain what's going on.

Essentially - we disallow discarding of data in mseal()'d mappings in
instances where the user couldn't otherwise write to that data.

We retain the existing behaviour here regarding MAP_PRIVATE mappings of
file-backed mappings, which entails some complexity - while this, strictly
speaking - appears to violate mseal() semantics, it may interact badly
with users which expect to be able to madvise(MADV_DONTNEED) .text
mappings for instance.

We may revisit this at a later date.

No functional change intended.

Link: https://lkml.kernel.org/r/492a98d9189646e92c8f23f4cce41ed323fe01df.1753431105.git.lorenzo.stoakes@oracle.com


Signed-off-by: default avatarLorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: default avatarLiam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: default avatarPedro Falcato <pfalcato@suse.de>
Acked-by: default avatarDavid Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jeff Xu <jeffxu@chromium.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent f225b34f

mm/madvise.c

+70 −1
Original line number Diff line number Diff line
@@ -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 +1257,74 @@ 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 VMA isn't sealed we're good. */

	if (can_modify_vma(vma))

		return true;



	/* For a sealed VMA, we only care about discard operations. */

	if (!is_discard(madv_behavior->behavior))

		return true;



	/*

	 * We explicitly permit all file-backed mappings, whether MAP_SHARED or

	 * MAP_PRIVATE.

	 *

	 * The latter causes some complications. Because now, one can mmap()

	 * read/write a MAP_PRIVATE mapping, write to it, then mprotect()

	 * read-only, mseal() and a discard will be permitted.

	 *

	 * However, in order to avoid issues with potential use of madvise(...,

	 * MADV_DONTNEED) of mseal()'d .text mappings we, for the time being,

	 * permit this.

	 */

	if (!vma_is_anonymous(vma))

		return true;



	/* If the user could write to the mapping anyway, then this is fine. */

	if ((vma->vm_flags & VM_WRITE) &&

	    arch_vma_access_permitted(vma, /* write= */ true,

			/* execute= */ false, /* foreign= */ false))

		return true;



	/* Otherwise, we are not permitted to perform this operation. */

	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 +1338,7 @@ 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) {

mm/mseal.c

+0 −49
Original line number Diff line number Diff line
@@ -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)

mm/vma.h

+0 −7
Original line number Diff line number Diff line
@@ -577,8 +577,6 @@ 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)
@@ -586,11 +584,6 @@ 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)