[PATCH v1 1/2] mseal: Two fixes for madvise(MADV_DONTNEED) when sealed

From: jeffxu
Date: Wed Oct 16 2024 - 20:51:31 EST


From: Jeff Xu <jeffxu@xxxxxxxxxxxx>

Two fixes for madvise(MADV_DONTNEED) when sealed.

For PROT_NONE mappings, the previous blocking of
madvise(MADV_DONTNEED) is unnecessary. As PROT_NONE already prohibits
memory access, madvise(MADV_DONTNEED) should be allowed to proceed in
order to free the page.

For file-backed, private, read-only memory mappings, we previously did
not block the madvise(MADV_DONTNEED). This was based on
the assumption that the memory's content, being file-backed, could be
retrieved from the file if accessed again. However, this assumption
failed to consider scenarios where a mapping is initially created as
read-write, modified, and subsequently changed to read-only. The newly
introduced VM_WASWRITE flag addresses this oversight.

Reported-by: Pedro Falcato <pedro.falcato@xxxxxxxxx>
Link:https://lore.kernel.org/all/CABi2SkW2XzuZ2-TunWOVzTEX1qc29LhjfNQ3hD4Nym8U-_f+ug@xxxxxxxxxxxxxx/
Fixes: 8be7258aad44 ("mseal: add mseal syscall")
Cc: <stable@xxxxxxxxxxxxxxx> # 6.11.y: 4d1b3416659b: mm: move can_modify_vma to mm/vma.h
Cc: <stable@xxxxxxxxxxxxxxx> # 6.11.y: 4a2dd02b0916: mm/mprotect: replace can_modify_mm with can_modify_vma
Cc: <stable@xxxxxxxxxxxxxxx> # 6.11.y: 23c57d1fa2b9: mseal: replace can_modify_mm_madv with a vma variant
Cc: <stable@xxxxxxxxxxxxxxx> # 6.11.y
Signed-off-by: Jeff Xu <jeffxu@xxxxxxxxxxxx>
---
include/linux/mm.h | 2 ++
mm/mprotect.c | 3 +++
mm/mseal.c | 42 ++++++++++++++++++++++++++++++++++++------
3 files changed, 41 insertions(+), 6 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 4c32003c8404..b402eca2565a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -430,6 +430,8 @@ extern unsigned int kobjsize(const void *objp);
#ifdef CONFIG_64BIT
/* VM is sealed, in vm_flags */
#define VM_SEALED _BITUL(63)
+/* VM was writable */
+#define VM_WASWRITE _BITUL(62)
#endif

/* Bits set in the VMA until the stack is in its final location */
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 0c5d6d06107d..6397135ca526 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -821,6 +821,9 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
break;
}

+ if ((vma->vm_flags & VM_WRITE) && !(newflags & VM_WRITE))
+ newflags |= VM_WASWRITE;
+
error = security_file_mprotect(vma, reqprot, prot);
if (error)
break;
diff --git a/mm/mseal.c b/mm/mseal.c
index ece977bd21e1..28f28487be17 100644
--- a/mm/mseal.c
+++ b/mm/mseal.c
@@ -36,12 +36,8 @@ static bool is_madv_discard(int behavior)
return false;
}

-static bool is_ro_anon(struct vm_area_struct *vma)
+static bool anon_is_ro(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.
@@ -53,6 +49,22 @@ static bool is_ro_anon(struct vm_area_struct *vma)
return false;
}

+static bool vma_is_prot_none(struct vm_area_struct *vma)
+{
+ if ((vma->vm_flags & VM_ACCESS_FLAGS) == VM_NONE)
+ return true;
+
+ return false;
+}
+
+static bool vma_was_writable_turn_readonly(struct vm_area_struct *vma)
+{
+ if (!(vma->vm_flags & VM_WRITE) && vma->vm_flags & VM_WASWRITE)
+ return true;
+
+ return false;
+}
+
/*
* Check if a vma is allowed to be modified by madvise.
*/
@@ -61,7 +73,25 @@ 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)))
+ /* not sealed */
+ if (likely(can_modify_vma(vma)))
+ return true;
+
+ /* PROT_NONE mapping */
+ if (vma_is_prot_none(vma))
+ return true;
+
+ /* file-backed private mapping */
+ if (vma->vm_file) {
+ /* read-only but was writeable */
+ if (vma_was_writable_turn_readonly(vma))
+ return false;
+
+ return true;
+ }
+
+ /* anonymous mapping is read-only */
+ if (anon_is_ro(vma))
return false;

/* Allow by default. */
--
2.47.0.rc1.288.g06298d1525-goog