| Message ID | 20210909162248.14969-5-david@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | s390: fixes, cleanups and optimizations for page table walkers | expand |
On Thu, 9 Sep 2021 18:22:43 +0200 David Hildenbrand <david@redhat.com> wrote: > There are multiple things broken about our storage key handling > functions: > > 1. We should not walk/touch page tables outside of VMA boundaries when > holding only the mmap sem in read mode. Evil user space can modify the > VMA layout just before this function runs and e.g., trigger races with > page table removal code since commit dd2283f2605e ("mm: mmap: zap pages > with read mmap_sem in munmap"). gfn_to_hva() will only translate using > KVM memory regions, but won't validate the VMA. > > 2. We should not allocate page tables outside of VMA boundaries: if > evil user space decides to map hugetlbfs to these ranges, bad things > will happen because we suddenly have PTE or PMD page tables where we > shouldn't have them. > > 3. We don't handle large PUDs that might suddenly appeared inside our page > table hierarchy. > > Don't manually allocate page tables, properly validate that we have VMA and > bail out on pud_large(). > > All callers of page table handling functions, except > get_guest_storage_key(), call fixup_user_fault() in case they > receive an -EFAULT and retry; this will allocate the necessary page tables > if required. > > To keep get_guest_storage_key() working as expected and not requiring > kvm_s390_get_skeys() to call fixup_user_fault() distinguish between > "there is simply no page table or huge page yet and the key is assumed > to be 0" and "this is a fault to be reported". > > Although commit 637ff9efe5ea ("s390/mm: Add huge pmd storage key handling") > introduced most of the affected code, it was actually already broken > before when using get_locked_pte() without any VMA checks. > > Note: Ever since commit 637ff9efe5ea ("s390/mm: Add huge pmd storage key > handling") we can no longer set a guest storage key (for example from > QEMU during VM live migration) without actually resolving a fault. > Although we would have created most page tables, we would choke on the > !pmd_present(), requiring a call to fixup_user_fault(). I would > have thought that this is problematic in combination with postcopy life > migration ... but nobody noticed and this patch doesn't change the > situation. So maybe it's just fine. > > Fixes: 9fcf93b5de06 ("KVM: S390: Create helper function get_guest_storage_key") > Fixes: 24d5dd0208ed ("s390/kvm: Provide function for setting the guest storage key") > Fixes: a7e19ab55ffd ("KVM: s390: handle missing storage-key facility") > Signed-off-by: David Hildenbrand <david@redhat.com> Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com> > --- > arch/s390/mm/pgtable.c | 57 +++++++++++++++++++++++++++++------------- > 1 file changed, 39 insertions(+), 18 deletions(-) > > diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c > index 54969e0f3a94..5fb409ff7842 100644 > --- a/arch/s390/mm/pgtable.c > +++ b/arch/s390/mm/pgtable.c > @@ -429,22 +429,36 @@ static inline pmd_t pmdp_flush_lazy(struct mm_struct *mm, > } > > #ifdef CONFIG_PGSTE > -static pmd_t *pmd_alloc_map(struct mm_struct *mm, unsigned long addr) > +static int pmd_lookup(struct mm_struct *mm, unsigned long addr, pmd_t **pmdp) > { > + struct vm_area_struct *vma; > pgd_t *pgd; > p4d_t *p4d; > pud_t *pud; > - pmd_t *pmd; > + > + /* We need a valid VMA, otherwise this is clearly a fault. */ > + vma = vma_lookup(mm, addr); > + if (!vma) > + return -EFAULT; > > pgd = pgd_offset(mm, addr); > - p4d = p4d_alloc(mm, pgd, addr); > - if (!p4d) > - return NULL; > - pud = pud_alloc(mm, p4d, addr); > - if (!pud) > - return NULL; > - pmd = pmd_alloc(mm, pud, addr); > - return pmd; > + if (!pgd_present(*pgd)) > + return -ENOENT; > + > + p4d = p4d_offset(pgd, addr); > + if (!p4d_present(*p4d)) > + return -ENOENT; > + > + pud = pud_offset(p4d, addr); > + if (!pud_present(*pud)) > + return -ENOENT; > + > + /* Large PUDs are not supported yet. */ > + if (pud_large(*pud)) > + return -EFAULT; > + > + *pmdp = pmd_offset(pud, addr); > + return 0; > } > #endif > > @@ -778,8 +792,7 @@ int set_guest_storage_key(struct mm_struct *mm, unsigned long addr, > pmd_t *pmdp; > pte_t *ptep; > > - pmdp = pmd_alloc_map(mm, addr); > - if (unlikely(!pmdp)) > + if (pmd_lookup(mm, addr, &pmdp)) > return -EFAULT; > > ptl = pmd_lock(mm, pmdp); > @@ -881,8 +894,7 @@ int reset_guest_reference_bit(struct mm_struct *mm, unsigned long addr) > pte_t *ptep; > int cc = 0; > > - pmdp = pmd_alloc_map(mm, addr); > - if (unlikely(!pmdp)) > + if (pmd_lookup(mm, addr, &pmdp)) > return -EFAULT; > > ptl = pmd_lock(mm, pmdp); > @@ -935,15 +947,24 @@ int get_guest_storage_key(struct mm_struct *mm, unsigned long addr, > pmd_t *pmdp; > pte_t *ptep; > > - pmdp = pmd_alloc_map(mm, addr); > - if (unlikely(!pmdp)) > + /* > + * If we don't have a PTE table and if there is no huge page mapped, > + * the storage key is 0. > + */ > + *key = 0; > + > + switch (pmd_lookup(mm, addr, &pmdp)) { > + case -ENOENT: > + return 0; > + case 0: > + break; > + default: > return -EFAULT; > + } > > ptl = pmd_lock(mm, pmdp); > if (!pmd_present(*pmdp)) { > - /* Not yet mapped memory has a zero key */ > spin_unlock(ptl); > - *key = 0; > return 0; > } >
diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c index 54969e0f3a94..5fb409ff7842 100644 --- a/arch/s390/mm/pgtable.c +++ b/arch/s390/mm/pgtable.c @@ -429,22 +429,36 @@ static inline pmd_t pmdp_flush_lazy(struct mm_struct *mm, } #ifdef CONFIG_PGSTE -static pmd_t *pmd_alloc_map(struct mm_struct *mm, unsigned long addr) +static int pmd_lookup(struct mm_struct *mm, unsigned long addr, pmd_t **pmdp) { + struct vm_area_struct *vma; pgd_t *pgd; p4d_t *p4d; pud_t *pud; - pmd_t *pmd; + + /* We need a valid VMA, otherwise this is clearly a fault. */ + vma = vma_lookup(mm, addr); + if (!vma) + return -EFAULT; pgd = pgd_offset(mm, addr); - p4d = p4d_alloc(mm, pgd, addr); - if (!p4d) - return NULL; - pud = pud_alloc(mm, p4d, addr); - if (!pud) - return NULL; - pmd = pmd_alloc(mm, pud, addr); - return pmd; + if (!pgd_present(*pgd)) + return -ENOENT; + + p4d = p4d_offset(pgd, addr); + if (!p4d_present(*p4d)) + return -ENOENT; + + pud = pud_offset(p4d, addr); + if (!pud_present(*pud)) + return -ENOENT; + + /* Large PUDs are not supported yet. */ + if (pud_large(*pud)) + return -EFAULT; + + *pmdp = pmd_offset(pud, addr); + return 0; } #endif @@ -778,8 +792,7 @@ int set_guest_storage_key(struct mm_struct *mm, unsigned long addr, pmd_t *pmdp; pte_t *ptep; - pmdp = pmd_alloc_map(mm, addr); - if (unlikely(!pmdp)) + if (pmd_lookup(mm, addr, &pmdp)) return -EFAULT; ptl = pmd_lock(mm, pmdp); @@ -881,8 +894,7 @@ int reset_guest_reference_bit(struct mm_struct *mm, unsigned long addr) pte_t *ptep; int cc = 0; - pmdp = pmd_alloc_map(mm, addr); - if (unlikely(!pmdp)) + if (pmd_lookup(mm, addr, &pmdp)) return -EFAULT; ptl = pmd_lock(mm, pmdp); @@ -935,15 +947,24 @@ int get_guest_storage_key(struct mm_struct *mm, unsigned long addr, pmd_t *pmdp; pte_t *ptep; - pmdp = pmd_alloc_map(mm, addr); - if (unlikely(!pmdp)) + /* + * If we don't have a PTE table and if there is no huge page mapped, + * the storage key is 0. + */ + *key = 0; + + switch (pmd_lookup(mm, addr, &pmdp)) { + case -ENOENT: + return 0; + case 0: + break; + default: return -EFAULT; + } ptl = pmd_lock(mm, pmdp); if (!pmd_present(*pmdp)) { - /* Not yet mapped memory has a zero key */ spin_unlock(ptl); - *key = 0; return 0; }
There are multiple things broken about our storage key handling functions: 1. We should not walk/touch page tables outside of VMA boundaries when holding only the mmap sem in read mode. Evil user space can modify the VMA layout just before this function runs and e.g., trigger races with page table removal code since commit dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap"). gfn_to_hva() will only translate using KVM memory regions, but won't validate the VMA. 2. We should not allocate page tables outside of VMA boundaries: if evil user space decides to map hugetlbfs to these ranges, bad things will happen because we suddenly have PTE or PMD page tables where we shouldn't have them. 3. We don't handle large PUDs that might suddenly appeared inside our page table hierarchy. Don't manually allocate page tables, properly validate that we have VMA and bail out on pud_large(). All callers of page table handling functions, except get_guest_storage_key(), call fixup_user_fault() in case they receive an -EFAULT and retry; this will allocate the necessary page tables if required. To keep get_guest_storage_key() working as expected and not requiring kvm_s390_get_skeys() to call fixup_user_fault() distinguish between "there is simply no page table or huge page yet and the key is assumed to be 0" and "this is a fault to be reported". Although commit 637ff9efe5ea ("s390/mm: Add huge pmd storage key handling") introduced most of the affected code, it was actually already broken before when using get_locked_pte() without any VMA checks. Note: Ever since commit 637ff9efe5ea ("s390/mm: Add huge pmd storage key handling") we can no longer set a guest storage key (for example from QEMU during VM live migration) without actually resolving a fault. Although we would have created most page tables, we would choke on the !pmd_present(), requiring a call to fixup_user_fault(). I would have thought that this is problematic in combination with postcopy life migration ... but nobody noticed and this patch doesn't change the situation. So maybe it's just fine. Fixes: 9fcf93b5de06 ("KVM: S390: Create helper function get_guest_storage_key") Fixes: 24d5dd0208ed ("s390/kvm: Provide function for setting the guest storage key") Fixes: a7e19ab55ffd ("KVM: s390: handle missing storage-key facility") Signed-off-by: David Hildenbrand <david@redhat.com> --- arch/s390/mm/pgtable.c | 57 +++++++++++++++++++++++++++++------------- 1 file changed, 39 insertions(+), 18 deletions(-)