[RFC PATCH v1 2/2] mseal: allow noop mprotect

[jeffxu@chromium.org]

From: Jeff Xu <jeffxu@chromium.org>

Initially, when mseal was introduced in 6.10, semantically, when a VMA
within the specified address range is sealed, the mprotect will be rejected,
leaving all of VMA unmodified. However, adding an extra loop to check the mseal
flag for every VMA slows things down a bit, therefore in 6.12, this issue was
solved by removing can_modify_mm and checking each VMA’s mseal flag directly
without an extra loop [1]. This is a semantic change, i.e. partial update is
allowed, VMAs can be updated until a sealed VMA is found.

The new semantic also means, we could allow mprotect on a sealed VMA if the new
attribute of VMA remains the same as the old one. Relaxing this avoids unnecessary
impacts for applications that want to seal a particular mapping. Doing this also
has no security impact.

[1] https://lore.kernel.org/all/20240817-mseal-depessimize-v3-0-d8d2e037df30@gmail.com/

Fixes: 4a2dd02b0916 ("mm/mprotect: replace can_modify_mm with can_modify_vma")
Signed-off-by: Jeff Xu <jeffxu@chromium.org>
---
 mm/mprotect.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 516b1d847e2c..a24d23967aa5 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -613,14 +613,14 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
 	unsigned long charged = 0;
 	int error;
 
-	if (!can_modify_vma(vma))
-		return -EPERM;
-
 	if (newflags == oldflags) {
 		*pprev = vma;
 		return 0;
 	}
 
+	if (!can_modify_vma(vma))
+		return -EPERM;
+
 	/*
 	 * Do PROT_NONE PFN permission checks here when we can still
 	 * bail out without undoing a lot of state. This is a rather
-- 
2.49.0.rc0.332.g42c0ae87b1-goog