arm64: pageattr: Explicitly bail out when changing permissions for vmalloc_huge mappings

arm64 uses apply_to_page_range to change permissions for kernel vmalloc mappings, which does not support changing permissions for block mappings. This function will change permissions until it encounters a block mapping, and will bail out with a warning. Since there are no reports of this triggering, it implies that there are currently no cases of code doing a vmalloc_huge() followed by partial permission change. But this is a footgun waiting to go off, so let's detect it early and avoid the possibility of permissions in an intermediate state. So, explicitly disallow changing permissions for VM_ALLOW_HUGE_VMAP mappings.

Reviewed-by Ryan Roberts <[email protected]>

Reviewed-by Mike Rapoport (Microsoft) <[email protected]>

Signed-off-by Dev Jain <[email protected]>

---
v1->v2:
- Improve changelog, keep mention of page mappings in comment

 arch/arm64/mm/pageattr.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

arch/arm64/mm/pageattr.c +2 -2

index 39fd1f7ff02a..04d4a8f676db 100644
@@ -96,8 +96,8 @@ static int change_memory_common(unsigned long addr, int numpages,
 	 * we are operating on does not result in such splitting.
 	 *
 	 * Let's restrict ourselves to mappings created by vmalloc (or vmap).
-	 * Those are guaranteed to consist entirely of page mappings, and
-	 * splitting is never needed.
+	 * Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
+	 * mappings are updated and splitting is never needed.
 	 *
 	 * So check whether the [addr, addr + size) interval is entirely
 	 * covered by precisely one VM area that has the VM_ALLOC flag set.

arm64 uses apply_to_page_range to change permissions for kernel vmalloc mappings, which does not support changing permissions for block mappings. This function will change permissions until it encounters a block mapping, and will bail out with a warning. Since there are no reports of this triggering, it implies that there are currently no cases of code doing a vmalloc_huge() followed by partial permission change. But this is a footgun waiting to go off, so let's detect it early and avoid the possibility of permissions in an intermediate state. So, explicitly disallow changing permissions for VM_ALLOW_HUGE_VMAP mappings.

Reviewed-by Ryan Roberts <[email protected]>

Reviewed-by Mike Rapoport (Microsoft) <[email protected]>

Signed-off-by Dev Jain <[email protected]>

---
v1->v2:
- Improve changelog, keep mention of page mappings in comment

  arch/arm64/mm/pageattr.c | 6 +++---
  1 file changed, 3 insertions(+), 3 deletions(-)

arm64 uses apply_to_page_range to change permissions for kernel vmalloc mappings, which does not support changing permissions for block mappings. This function will change permissions until it encounters a block mapping, and will bail out with a warning. Since there are no reports of this triggering, it implies that there are currently no cases of code doing a vmalloc_huge() followed by partial permission change. But this is a footgun waiting to go off, so let's detect it early and avoid the possibility of permissions in an intermediate state. So, explicitly disallow changing permissions for VM_ALLOW_HUGE_VMAP mappings.

Reviewed-by Ryan Roberts <[email protected]>

Reviewed-by Mike Rapoport (Microsoft) <[email protected]>

Signed-off-by Dev Jain <[email protected]>

---
v1->v2:
- Improve changelog, keep mention of page mappings in comment

  arch/arm64/mm/pageattr.c | 6 +++---
  1 file changed, 3 insertions(+), 3 deletions(-)

arch/arm64/mm/pageattr.c +2 -2

index 39fd1f7ff02a..04d4a8f676db 100644
@@ -96,8 +96,8 @@ static int change_memory_common(unsigned long addr, int numpages,
  	 * we are operating on does not result in such splitting.
  	 *
  	 * Let's restrict ourselves to mappings created by vmalloc (or vmap).
-	 * Those are guaranteed to consist entirely of page mappings, and
-	 * splitting is never needed.
+	 * Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
+	 * mappings are updated and splitting is never needed.
  	 *
  	 * So check whether the [addr, addr + size) interval is entirely
  	 * covered by precisely one VM area that has the VM_ALLOC flag set.

Quoted 40 lines · Dev Jain

arm64 uses apply_to_page_range to change permissions for kernel vmalloc mappings, which does not support changing permissions for block mappings. This function will change permissions until it encounters a block mapping, and will bail out with a warning. Since there are no reports of this triggering, it implies that there are currently no cases of code doing a vmalloc_huge() followed by partial permission change. But this is a footgun waiting to go off, so let's detect it early and avoid the possibility of permissions in an intermediate state. So,  explicitly disallow changing permissions for VM_ALLOW_HUGE_VMAP mappings.

Reviewed-by Ryan Roberts <[email protected]>

Reviewed-by Mike Rapoport (Microsoft) <[email protected]>

Signed-off-by Dev Jain <[email protected]>

---
v1->v2:
  - Improve changelog, keep mention of page mappings in comment

  arch/arm64/mm/pageattr.c | 6 +++---

  1 file changed, 3 insertions(+), 3 deletions(-)

arch/arm64/mm/pageattr.c +2 -2

index 39fd1f7ff02a..04d4a8f676db 100644
@@ -96,8 +96,8 @@ static int change_memory_common(unsigned long addr, int
 numpages,
        * we are operating on does not result in such splitting.
        *
        * Let's restrict ourselves to mappings created by vmalloc (or vmap).
-     * Those are guaranteed to consist entirely of page mappings, and
-     * splitting is never needed.
+     * Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
+     * mappings are updated and splitting is never needed.
        *
        * So check whether the [addr, addr + size) interval is entirely

       * covered by precisely one VM area that has the VM_ALLOC flag set.

arch/arm64/mm/pageattr.c +0 -0

@@ -105,7 +105,7 @@ static int change_memory_common(unsigned long addr, int
 numpages,
       area = find_vm_area((void *)addr);

      if (!area ||

Quoted 5 lines

          end > (unsigned long)kasan_reset_tag(area->addr) + area->size || -        !(area->flags & VM_ALLOC))
+        ((area->flags & (VM_ALLOC | VM_ALLOW_HUGE_VMAP)) != VM_ALLOC)) return -EINVAL;
        if (!numpages)

Makes sense to me. Whenever required, we can improve the checks (or even try supporting splitting).

Acked-by David Hildenbrand <[email protected]>

arm64 uses apply_to_page_range to change permissions for kernel vmalloc mappings, which does not support changing permissions for block mappings. This function will change permissions until it encounters a block mapping, and will bail out with a warning. Since there are no reports of this triggering, it implies that there are currently no cases of code doing a vmalloc_huge() followed by partial permission change. But this is a footgun waiting to go off, so let's detect it early and avoid the possibility of permissions in an intermediate state. So, explicitly disallow changing permissions for VM_ALLOW_HUGE_VMAP mappings.

Reviewed-by Ryan Roberts <[email protected]>

Reviewed-by Mike Rapoport (Microsoft) <[email protected]>

Signed-off-by Dev Jain <[email protected]>

---
v1->v2:
- Improve changelog, keep mention of page mappings in comment

  arch/arm64/mm/pageattr.c | 6 +++---
  1 file changed, 3 insertions(+), 3 deletions(-)

arch/arm64/mm/pageattr.c +3 -3

index 39fd1f7ff02a..04d4a8f676db 100644
@@ -96,8 +96,8 @@ static int change_memory_common(unsigned long addr, int numpages,
  	 * we are operating on does not result in such splitting.
  	 *
  	 * Let's restrict ourselves to mappings created by vmalloc (or vmap).
-	 * Those are guaranteed to consist entirely of page mappings, and
-	 * splitting is never needed.
+	 * Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
+	 * mappings are updated and splitting is never needed.
  	 *
  	 * So check whether the [addr, addr + size) interval is entirely
  	 * covered by precisely one VM area that has the VM_ALLOC flag set.

@@ -105,7 +105,7 @@ static int change_memory_common(unsigned long addr, int numpages,
  	area = find_vm_area((void *)addr);
  	if (!area ||
  	    end > (unsigned long)kasan_reset_tag(area->addr) + area->size ||
-	    !(area->flags & VM_ALLOC))
+	    ((area->flags & (VM_ALLOC | VM_ALLOW_HUGE_VMAP)) != VM_ALLOC))
  		return -EINVAL;
 
  	if (!numpages)

arm64 uses apply_to_page_range to change permissions for kernel vmalloc mappings, which does not support changing permissions for block mappings. This function will change permissions until it encounters a block mapping, and will bail out with a warning. Since there are no reports of this triggering, it implies that there are currently no cases of code doing a vmalloc_huge() followed by partial permission change. But this is a footgun waiting to go off, so let's detect it early and avoid the possibility of permissions in an intermediate state. So, explicitly disallow changing permissions for VM_ALLOW_HUGE_VMAP mappings.

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