| Message ID | 20240320001542.3203871-2-seanjc@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: Fix for a mostly benign gpc WARN | expand |
On Tue, 2024-03-19 at 17:15 -0700, Sean Christopherson wrote: > Add a helper to check that the incoming length for a gfn_to_pfn_cache is > valid with respect to the cache's GPA and/or HVA. To avoid activating a > cache with a bogus GPA, a future fix will fork the page split check in > the inner refresh path into activate() and the public rerfresh() APIs, at > which point KVM will check the length in three separate places. > > Deliberately keep the "page offset" logic open coded, as the only other > path that consumes the offset, __kvm_gpc_refresh(), already needs to > differentiate between GPA-based and HVA-based caches, and it's not obvious > that using a helper is a net positive in overall code readability. > > Note, for GPA-based caches, this has a subtle side effect of using the GPA > instead of the resolved HVA in the check() path, but that should be a nop > as the HVA offset is derived from the GPA, i.e. the two offsets are > identical, barring a KVM bug. > > Signed-off-by: Sean Christopherson <seanjc@google.com> Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
On 20/03/2024 00:15, Sean Christopherson wrote: > Add a helper to check that the incoming length for a gfn_to_pfn_cache is > valid with respect to the cache's GPA and/or HVA. To avoid activating a > cache with a bogus GPA, a future fix will fork the page split check in > the inner refresh path into activate() and the public rerfresh() APIs, at nit: typo > which point KVM will check the length in three separate places. > > Deliberately keep the "page offset" logic open coded, as the only other > path that consumes the offset, __kvm_gpc_refresh(), already needs to > differentiate between GPA-based and HVA-based caches, and it's not obvious > that using a helper is a net positive in overall code readability. > > Note, for GPA-based caches, this has a subtle side effect of using the GPA > instead of the resolved HVA in the check() path, but that should be a nop > as the HVA offset is derived from the GPA, i.e. the two offsets are > identical, barring a KVM bug. > > Signed-off-by: Sean Christopherson <seanjc@google.com> > --- > virt/kvm/pfncache.c | 27 +++++++++++++++++++-------- > 1 file changed, 19 insertions(+), 8 deletions(-) > Reviewed-by: Paul Durrant <paul@xen.org>
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c index 4e07112a24c2..8f2121b5f2a0 100644 --- a/virt/kvm/pfncache.c +++ b/virt/kvm/pfncache.c @@ -57,6 +57,19 @@ void gfn_to_pfn_cache_invalidate_start(struct kvm *kvm, unsigned long start, spin_unlock(&kvm->gpc_lock); } +static bool kvm_gpc_is_valid_len(gpa_t gpa, unsigned long uhva, + unsigned long len) +{ + unsigned long offset = kvm_is_error_gpa(gpa) ? offset_in_page(uhva) : + offset_in_page(gpa); + + /* + * The cached access must fit within a single page. The 'len' argument + * to activate() and refresh() exists only to enforce that. + */ + return offset + len <= PAGE_SIZE; +} + bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len) { struct kvm_memslots *slots = kvm_memslots(gpc->kvm); @@ -74,7 +87,7 @@ bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len) if (kvm_is_error_hva(gpc->uhva)) return false; - if (offset_in_page(gpc->uhva) + len > PAGE_SIZE) + if (!kvm_gpc_is_valid_len(gpc->gpa, gpc->uhva, len)) return false; if (!gpc->valid) @@ -247,13 +260,7 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l if (WARN_ON_ONCE(kvm_is_error_gpa(gpa) == kvm_is_error_hva(uhva))) return -EINVAL; - /* - * The cached acces must fit within a single page. The 'len' argument - * exists only to enforce that. - */ - page_offset = kvm_is_error_gpa(gpa) ? offset_in_page(uhva) : - offset_in_page(gpa); - if (page_offset + len > PAGE_SIZE) + if (!kvm_gpc_is_valid_len(gpa, uhva, len)) return -EINVAL; lockdep_assert_held(&gpc->refresh_lock); @@ -270,6 +277,8 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l old_uhva = PAGE_ALIGN_DOWN(gpc->uhva); if (kvm_is_error_gpa(gpa)) { + page_offset = offset_in_page(uhva); + gpc->gpa = INVALID_GPA; gpc->memslot = NULL; gpc->uhva = PAGE_ALIGN_DOWN(uhva); @@ -279,6 +288,8 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l } else { struct kvm_memslots *slots = kvm_memslots(gpc->kvm); + page_offset = offset_in_page(gpa); + if (gpc->gpa != gpa || gpc->generation != slots->generation || kvm_is_error_hva(gpc->uhva)) { gfn_t gfn = gpa_to_gfn(gpa);
Add a helper to check that the incoming length for a gfn_to_pfn_cache is valid with respect to the cache's GPA and/or HVA. To avoid activating a cache with a bogus GPA, a future fix will fork the page split check in the inner refresh path into activate() and the public rerfresh() APIs, at which point KVM will check the length in three separate places. Deliberately keep the "page offset" logic open coded, as the only other path that consumes the offset, __kvm_gpc_refresh(), already needs to differentiate between GPA-based and HVA-based caches, and it's not obvious that using a helper is a net positive in overall code readability. Note, for GPA-based caches, this has a subtle side effect of using the GPA instead of the resolved HVA in the check() path, but that should be a nop as the HVA offset is derived from the GPA, i.e. the two offsets are identical, barring a KVM bug. Signed-off-by: Sean Christopherson <seanjc@google.com> --- virt/kvm/pfncache.c | 27 +++++++++++++++++++-------- 1 file changed, 19 insertions(+), 8 deletions(-)