linux-kernel - [PATCH 6/7] mseal: Replace can_modify_mm

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Message-ID: <20240806212808.1885309-7-pedro.falcato@gmail.com>
Date: Tue,  6 Aug 2024 22:28:07 +0100
From: Pedro Falcato <pedro.falcato@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	oliver.sang@...el.com,
	torvalds@...ux-foundation.org,
	jeffxu@...gle.com,
	Michael Ellerman <mpe@...erman.id.au>,
	Pedro Falcato <pedro.falcato@...il.com>
Subject: [PATCH 6/7] mseal: Replace can_modify_mm_madv with a vma variant

Replace can_modify_mm_madv() with a single vma variant, and associated
checks in madvise.

While we're at it, also invert the order of checks in:
 if (unlikely(is_ro_anon(vma) && !can_modify_vma(vma))

Checking if we can modify the vma itself (through vm_flags) is
certainly cheaper than is_ro_anon() due to arch_vma_access_permitted()
looking at e.g pkeys registers (with extra branches) in some
architectures.

Signed-off-by: Pedro Falcato <pedro.falcato@...il.com>
---
 mm/internal.h |  6 ++----
 mm/madvise.c  | 13 +++----------
 mm/mseal.c    | 17 ++++-------------
 3 files changed, 9 insertions(+), 27 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 09ea930540f..4b516618389 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1517,8 +1517,7 @@ static inline bool can_modify_vma(struct vm_area_struct *vma)
 
 bool can_modify_mm(struct mm_struct *mm, unsigned long start,
 		unsigned long end);
-bool can_modify_mm_madv(struct mm_struct *mm, unsigned long start,
-		unsigned long end, int behavior);
+bool can_modify_vma_madv(struct vm_area_struct *vma, int behavior);
 #else
 static inline int can_do_mseal(unsigned long flags)
 {
@@ -1531,8 +1530,7 @@ static inline bool can_modify_mm(struct mm_struct *mm, unsigned long start,
 	return true;
 }
 
-static inline bool can_modify_mm_madv(struct mm_struct *mm, unsigned long start,
-		unsigned long end, int behavior)
+static inline bool can_modify_vma_madv(struct vm_area_struct *vma, int behavior)
 {
 	return true;
 }
diff --git a/mm/madvise.c b/mm/madvise.c
index 89089d84f8d..4e64770be16 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1031,6 +1031,9 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
 	struct anon_vma_name *anon_name;
 	unsigned long new_flags = vma->vm_flags;
 
+	if (unlikely(!can_modify_vma_madv(vma, behavior)))
+		return -EPERM;
+
 	switch (behavior) {
 	case MADV_REMOVE:
 		return madvise_remove(vma, prev, start, end);
@@ -1448,15 +1451,6 @@ int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int beh
 	start = untagged_addr_remote(mm, start);
 	end = start + len;
 
-	/*
-	 * Check if the address range is sealed for do_madvise().
-	 * can_modify_mm_madv assumes we have acquired the lock on MM.
-	 */
-	if (unlikely(!can_modify_mm_madv(mm, start, end, behavior))) {
-		error = -EPERM;
-		goto out;
-	}
-
 	blk_start_plug(&plug);
 	switch (behavior) {
 	case MADV_POPULATE_READ:
@@ -1470,7 +1464,6 @@ int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int beh
 	}
 	blk_finish_plug(&plug);
 
-out:
 	if (write)
 		mmap_write_unlock(mm);
 	else
diff --git a/mm/mseal.c b/mm/mseal.c
index 2170e2139ca..fdd1666344f 100644
--- a/mm/mseal.c
+++ b/mm/mseal.c
@@ -75,24 +75,15 @@ bool can_modify_mm(struct mm_struct *mm, unsigned long start, unsigned long end)
 }
 
 /*
- * Check if the vmas of a memory range are allowed to be modified by madvise.
- * the memory ranger can have a gap (unallocated memory).
- * return true, if it is allowed.
+ * Check if a vma is allowed to be modified by madvise.
  */
-bool can_modify_mm_madv(struct mm_struct *mm, unsigned long start, unsigned long end,
-		int behavior)
+bool can_modify_vma_madv(struct vm_area_struct *vma, int behavior)
 {
-	struct vm_area_struct *vma;
-
-	VMA_ITERATOR(vmi, mm, start);
-
 	if (!is_madv_discard(behavior))
 		return true;
 
-	/* going through each vma to check. */
-	for_each_vma_range(vmi, vma, end)
-		if (unlikely(is_ro_anon(vma) && !can_modify_vma(vma)))
-			return false;
+	if (unlikely(!can_modify_vma(vma) && is_ro_anon(vma)))
+		return false;
 
 	/* Allow by default. */
 	return true;
-- 
2.46.0