Clear to the @pages array pointer in sev_unpin_memory to avoid leaving a
dangling pointer to invalid memory.
Signed-off-by: Peter Gonda <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Tom Lendacky <[email protected]>
Cc: [email protected]
Cc: [email protected]
---
arch/x86/kvm/svm/sev.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 309bcdb2f929..485ad86c01c6 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -452,6 +452,7 @@ static void sev_unpin_memory(struct kvm *kvm, struct page **pages,
unpin_user_pages(pages, npages);
kvfree(pages);
sev->pages_locked -= npages;
+ *pages = NULL;
}
static void sev_clflush_pages(struct page *pages[], unsigned long npages)
--
2.37.0.rc0.161.g10f37bed90-goog
On Mon, Jun 27, 2022, Peter Gonda wrote:
> Clear to the @pages array pointer in sev_unpin_memory to avoid leaving a
> dangling pointer to invalid memory.
>
> Signed-off-by: Peter Gonda <[email protected]>
> Cc: Greg Thelen <[email protected]>
> Cc: Paolo Bonzini <[email protected]>
> Cc: Sean Christopherson <[email protected]>
> Cc: Tom Lendacky <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> ---
> arch/x86/kvm/svm/sev.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 309bcdb2f929..485ad86c01c6 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -452,6 +452,7 @@ static void sev_unpin_memory(struct kvm *kvm, struct page **pages,
> unpin_user_pages(pages, npages);
> kvfree(pages);
> sev->pages_locked -= npages;
> + *pages = NULL;
Would this have helped detect a real bug? I generally like cleaning up, but this
leaves things in a somewhat inconsistent state, e.g. when unpinning a kvm_enc_region,
pages will be NULL but npages will be non-zero. It's somewhat moot because the
region is immediately freed in that case, but that begs the question of what real
benefit this provides. sev_dbg_crypt() is the only flow where there's much danger
of a use-after-free.
> }
>
> static void sev_clflush_pages(struct page *pages[], unsigned long npages)
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>
On Mon, Jun 27, 2022 at 12:10 PM Sean Christopherson <[email protected]> wrote:
>
> On Mon, Jun 27, 2022, Peter Gonda wrote:
> > Clear to the @pages array pointer in sev_unpin_memory to avoid leaving a
> > dangling pointer to invalid memory.
> >
> > Signed-off-by: Peter Gonda <[email protected]>
> > Cc: Greg Thelen <[email protected]>
> > Cc: Paolo Bonzini <[email protected]>
> > Cc: Sean Christopherson <[email protected]>
> > Cc: Tom Lendacky <[email protected]>
> > Cc: [email protected]
> > Cc: [email protected]
> > ---
> > arch/x86/kvm/svm/sev.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > index 309bcdb2f929..485ad86c01c6 100644
> > --- a/arch/x86/kvm/svm/sev.c
> > +++ b/arch/x86/kvm/svm/sev.c
> > @@ -452,6 +452,7 @@ static void sev_unpin_memory(struct kvm *kvm, struct page **pages,
> > unpin_user_pages(pages, npages);
> > kvfree(pages);
> > sev->pages_locked -= npages;
> > + *pages = NULL;
>
> Would this have helped detect a real bug? I generally like cleaning up, but this
> leaves things in a somewhat inconsistent state, e.g. when unpinning a kvm_enc_region,
> pages will be NULL but npages will be non-zero. It's somewhat moot because the
> region is immediately freed in that case, but that begs the question of what real
> benefit this provides. sev_dbg_crypt() is the only flow where there's much danger
> of a use-after-free.
>
No strong opinion here, I just thought since this is a helper that
takes a 'struct page **pages" we may as well clear this. While there
are no bugs caught now if someone were to introduce something wrong
this would make it more clear.
We could update sev_unpin_memory() to take a int *npages so it can be
cleared as well. Since kvm_enc_region describes a region with u64
instead of pointers that seemed "safer" give you'd have to cast them
to dereference.
Totally fine with the NACK of course. =]
On Mon, Jun 27, 2022 at 1:37 PM Sean Christopherson <[email protected]> wrote:
>
> On Mon, Jun 27, 2022, Peter Gonda wrote:
> > Clear to the @pages array pointer in sev_unpin_memory to avoid leaving a
> > dangling pointer to invalid memory.
> >
> > Signed-off-by: Peter Gonda <[email protected]>
> > Cc: Greg Thelen <[email protected]>
> > Cc: Paolo Bonzini <[email protected]>
> > Cc: Sean Christopherson <[email protected]>
> > Cc: Tom Lendacky <[email protected]>
> > Cc: [email protected]
> > Cc: [email protected]
> > ---
> > arch/x86/kvm/svm/sev.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > index 309bcdb2f929..485ad86c01c6 100644
> > --- a/arch/x86/kvm/svm/sev.c
> > +++ b/arch/x86/kvm/svm/sev.c
> > @@ -452,6 +452,7 @@ static void sev_unpin_memory(struct kvm *kvm, struct page **pages,
> > unpin_user_pages(pages, npages);
> > kvfree(pages);
> > sev->pages_locked -= npages;
> > + *pages = NULL;
>
> Would this have helped detect a real bug? I generally like cleaning up, but this
> leaves things in a somewhat inconsistent state, e.g. when unpinning a kvm_enc_region,
> pages will be NULL but npages will be non-zero. It's somewhat moot because the
> region is immediately freed in that case, but that begs the question of what real
> benefit this provides. sev_dbg_crypt() is the only flow where there's much danger
> of a use-after-free.
+1
We could send thousands of patches like this. But I think none of them
would be directly helpful. Sometimes, things could be even worse,
i.e., imagine when those NULL pointers got deferenced, they become
BUG_ON(true).
If there are bugs related with those pointers, better to fix those bugs instead.
Thanks.
-Mingwei