On Wed, Mar 12, 2025 at 12:21:17AM +0000, jeffxu@chromium.org wrote:
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@gma...
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
Hm I'm not so sure about this, to me a seal means 'don't touch', even if the touch would be a no-op. It's simpler to be totally consistent on this and makes the code easier everywhere.
Because if we start saying 'apply mseal rules, except if we can determine this to be a no-op' then that implies we might have some inconsistency in other operations that do not do that, and sometimes a 'no-op' might be ill-defined etc.
I think generally I'd rather leave things as they are unless you have a specific real-life case where this is causing problems?