Re: [PATCH] x86/pat: fix W^X violation false-positives when running as Xen PV guest

From: Ingo Molnar
Date: Wed Apr 10 2024 - 10:01:41 EST

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* Juergen Gross <jgross@xxxxxxxx> wrote:

> When running as Xen PV guest in some cases W^X violation WARN()s have
> been observed. Those WARN()s are produced by verify_rwx(), which looks
> into the PTE to verify that writable kernel pages have the NX bit set
> in order to avoid code modifications of the kernel by rogue code.
>
> As the NX bits of all levels of translation entries are or-ed and the
> RW bits of all levels are and-ed, looking just into the PTE isn't enough
> for the decision that a writable page is executable, too. When running
> as a Xen PV guest, kernel initialization will set the NX bit in PMD
> entries of the initial page tables covering the .data segment.
>
> When finding the PTE to have set the RW bit but no NX bit, higher level
> entries must be looked at. Only when all levels have the RW bit set and
> no NX bit set, the W^X violation should be flagged.
>
> Additionally show_fault_oops() has a similar problem: it will issue the
> "kernel tried to execute NX-protected page" message only if it finds
> the NX bit set in the leaf translation entry, while any NX bit in
> non-leaf entries are being ignored for issuing the message.
>
> Modify lookup_address_in_pgd() to return the effective NX and RW bit
> values of the non-leaf translation entries and evaluate those as well
> in verify_rwx() and show_fault_oops().

Ok, this fix makes sense, as that's how the hardware works and we interpret
the pagetables poorly.

> Fixes: 652c5bf380ad ("x86/mm: Refuse W^X violations")
> Reported-by: Jason Andryuk <jandryuk@xxxxxxxxx>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> arch/x86/include/asm/pgtable_types.h | 2 +-
> arch/x86/kernel/sev.c | 3 +-
> arch/x86/mm/fault.c | 7 ++--
> arch/x86/mm/pat/set_memory.c | 56 +++++++++++++++++++++-------
> arch/x86/virt/svm/sev.c | 3 +-
> 5 files changed, 52 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 0b748ee16b3d..91ab538d3872 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -565,7 +565,7 @@ static inline void update_page_count(int level, unsigned long pages) { }
> */
> extern pte_t *lookup_address(unsigned long address, unsigned int *level);
> extern pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
> - unsigned int *level);
> + unsigned int *level, bool *nx, bool *rw);
> extern pmd_t *lookup_pmd_address(unsigned long address);
> extern phys_addr_t slow_virt_to_phys(void *__address);
> extern int __init kernel_map_pages_in_pgd(pgd_t *pgd, u64 pfn,

Please introduce a new lookup_address_in_pgd_attr() function or so, which
is used by code intentionally.

This avoids changing the arch/x86/kernel/sev.c and arch/x86/virt/svm/sev.c
uses, that retrieve these attributes but don't do anything with them:

> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 38ad066179d8..adba581e999d 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -516,12 +516,13 @@ static enum es_result vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt
> unsigned long va = (unsigned long)vaddr;
> unsigned int level;
> phys_addr_t pa;
> + bool nx, rw;
> pgd_t *pgd;
> pte_t *pte;
>
> pgd = __va(read_cr3_pa());
> pgd = &pgd[pgd_index(va)];
> - pte = lookup_address_in_pgd(pgd, va, &level);
> + pte = lookup_address_in_pgd(pgd, va, &level, &nx, &rw);
> if (!pte) {
> ctxt->fi.vector = X86_TRAP_PF;
> ctxt->fi.cr2 = vaddr;
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 622d12ec7f08..eb8e897a5653 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -514,18 +514,19 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
>
> if (error_code & X86_PF_INSTR) {
> unsigned int level;
> + bool nx, rw;
> pgd_t *pgd;
> pte_t *pte;
>
> pgd = __va(read_cr3_pa());
> pgd += pgd_index(address);
>
> - pte = lookup_address_in_pgd(pgd, address, &level);
> + pte = lookup_address_in_pgd(pgd, address, &level, &nx, &rw);
>
> - if (pte && pte_present(*pte) && !pte_exec(*pte))
> + if (pte && pte_present(*pte) && (!pte_exec(*pte) || nx))
> pr_crit("kernel tried to execute NX-protected page - exploit attempt? (uid: %d)\n",
> from_kuid(&init_user_ns, current_uid()));
> - if (pte && pte_present(*pte) && pte_exec(*pte) &&
> + if (pte && pte_present(*pte) && pte_exec(*pte) && !nx &&
> (pgd_flags(*pgd) & _PAGE_USER) &&
> (__read_cr4() & X86_CR4_SMEP))
> pr_crit("unable to execute userspace code (SMEP?) (uid: %d)\n",

This should be a separate patch - as it might change observed behavior.

> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 80c9037ffadf..baa4dc4748e9 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -619,7 +619,8 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long start,
> * Validate strict W^X semantics.
> */
> static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long start,
> - unsigned long pfn, unsigned long npg)
> + unsigned long pfn, unsigned long npg,
> + bool nx, bool rw)
> {
> unsigned long end;
>
> @@ -641,6 +642,10 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
> if ((pgprot_val(new) & (_PAGE_RW | _PAGE_NX)) != _PAGE_RW)
> return new;
>
> + /* Non-leaf translation entries can disable writing or execution. */
> + if (!rw || nx)
> + return new;
> +
> end = start + npg * PAGE_SIZE - 1;
> WARN_ONCE(1, "CPA detected W^X violation: %016llx -> %016llx range: 0x%016lx - 0x%016lx PFN %lx\n",
> (unsigned long long)pgprot_val(old),
> @@ -660,17 +665,22 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
> * Return a pointer to the entry and the level of the mapping.
> */
> pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
> - unsigned int *level)
> + unsigned int *level, bool *nx, bool *rw)
> {
> p4d_t *p4d;
> pud_t *pud;
> pmd_t *pmd;
>
> *level = PG_LEVEL_NONE;
> + *nx = false;
> + *rw = true;
>
> if (pgd_none(*pgd))
> return NULL;
>
> + *nx |= pgd_flags(*pgd) & _PAGE_NX;
> + *rw &= pgd_flags(*pgd) & _PAGE_RW;
> +
> p4d = p4d_offset(pgd, address);
> if (p4d_none(*p4d))
> return NULL;
> @@ -679,6 +689,9 @@ pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
> if (p4d_leaf(*p4d) || !p4d_present(*p4d))
> return (pte_t *)p4d;
>
> + *nx |= p4d_flags(*p4d) & _PAGE_NX;
> + *rw &= p4d_flags(*p4d) & _PAGE_RW;
> +
> pud = pud_offset(p4d, address);
> if (pud_none(*pud))
> return NULL;
> @@ -687,6 +700,9 @@ pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
> if (pud_leaf(*pud) || !pud_present(*pud))
> return (pte_t *)pud;
>
> + *nx |= pud_flags(*pud) & _PAGE_NX;
> + *rw &= pud_flags(*pud) & _PAGE_RW;
> +
> pmd = pmd_offset(pud, address);
> if (pmd_none(*pmd))
> return NULL;
> @@ -695,6 +711,9 @@ pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
> if (pmd_leaf(*pmd) || !pmd_present(*pmd))
> return (pte_t *)pmd;
>
> + *nx |= pmd_flags(*pmd) & _PAGE_NX;
> + *rw &= pmd_flags(*pmd) & _PAGE_RW;
> +
> *level = PG_LEVEL_4K;
>

This should be a separate preparatory patch that also introduces the new
method - without changing any behavior.

return pte_offset_kernel(pmd, address);
> @@ -710,18 +729,24 @@ pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
> */
> pte_t *lookup_address(unsigned long address, unsigned int *level)
> {
> - return lookup_address_in_pgd(pgd_offset_k(address), address, level);
> + bool nx, rw;
> +
> + return lookup_address_in_pgd(pgd_offset_k(address), address, level,
> + &nx, &rw);
> }
> EXPORT_SYMBOL_GPL(lookup_address);
>
> static pte_t *_lookup_address_cpa(struct cpa_data *cpa, unsigned long address,
> - unsigned int *level)
> + unsigned int *level, bool *nx, bool *rw)
> {
> - if (cpa->pgd)
> - return lookup_address_in_pgd(cpa->pgd + pgd_index(address),
> - address, level);
> + pgd_t *pgd;
> +
> + if (!cpa->pgd)
> + pgd = pgd_offset_k(address);
> + else
> + pgd = cpa->pgd + pgd_index(address);
>
> - return lookup_address(address, level);
> + return lookup_address_in_pgd(pgd, address, level, nx, rw);

I think it would be better to split out this change as well into a separate
patch. It changes the flow from lookup_address_in_pgd() + lookup_address()
to only use lookup_address_in_pgd(), which is an identity transformation
that should be better done separately.

> }
>
> /*
> @@ -849,12 +874,13 @@ static int __should_split_large_page(pte_t *kpte, unsigned long address,
> pgprot_t old_prot, new_prot, req_prot, chk_prot;
> pte_t new_pte, *tmp;
> enum pg_level level;
> + bool nx, rw;
>
> /*
> * Check for races, another CPU might have split this page
> * up already:
> */
> - tmp = _lookup_address_cpa(cpa, address, &level);
> + tmp = _lookup_address_cpa(cpa, address, &level, &nx, &rw);
> if (tmp != kpte)
> return 1;
>
> @@ -965,7 +991,8 @@ static int __should_split_large_page(pte_t *kpte, unsigned long address,
> new_prot = static_protections(req_prot, lpaddr, old_pfn, numpages,
> psize, CPA_DETECT);
>
> - new_prot = verify_rwx(old_prot, new_prot, lpaddr, old_pfn, numpages);
> + new_prot = verify_rwx(old_prot, new_prot, lpaddr, old_pfn, numpages,
> + nx, rw);
>
> /*
> * If there is a conflict, split the large page.
> @@ -1046,6 +1073,7 @@ __split_large_page(struct cpa_data *cpa, pte_t *kpte, unsigned long address,
> pte_t *pbase = (pte_t *)page_address(base);
> unsigned int i, level;
> pgprot_t ref_prot;
> + bool nx, rw;
> pte_t *tmp;
>
> spin_lock(&pgd_lock);
> @@ -1053,7 +1081,7 @@ __split_large_page(struct cpa_data *cpa, pte_t *kpte, unsigned long address,
> * Check for races, another CPU might have split this page
> * up for us already:
> */
> - tmp = _lookup_address_cpa(cpa, address, &level);
> + tmp = _lookup_address_cpa(cpa, address, &level, &nx, &rw);
> if (tmp != kpte) {
> spin_unlock(&pgd_lock);
> return 1;
> @@ -1594,10 +1622,11 @@ static int __change_page_attr(struct cpa_data *cpa, int primary)
> int do_split, err;
> unsigned int level;
> pte_t *kpte, old_pte;
> + bool nx, rw;
>
> address = __cpa_addr(cpa, cpa->curpage);
> repeat:
> - kpte = _lookup_address_cpa(cpa, address, &level);
> + kpte = _lookup_address_cpa(cpa, address, &level, &nx, &rw);
> if (!kpte)
> return __cpa_process_fault(cpa, address, primary);
>
> @@ -1619,7 +1648,8 @@ static int __change_page_attr(struct cpa_data *cpa, int primary)
> new_prot = static_protections(new_prot, address, pfn, 1, 0,
> CPA_PROTECT);
>
> - new_prot = verify_rwx(old_prot, new_prot, address, pfn, 1);
> + new_prot = verify_rwx(old_prot, new_prot, address, pfn, 1,
> + nx, rw);
>
> new_prot = pgprot_clear_protnone_bits(new_prot);

And then this should be the final patch, which fixes RWX verification
within the CPA code.

Thanks,

Ingo

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