From fd25a67d92e44b61d05d92658b23d026202a1656 Mon Sep 17 00:00:00 2001 From: Juergen Gross To: x86@xxxxxxxxxx To: linux-kernel@xxxxxxxxxxxxxxx To: xen-devel@xxxxxxxxxxxxxxxxxxxx Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: Dave Hansen Cc: "H. Peter Anvin" Cc: Andy Lutomirski Cc: Peter Zijlstra Date: Thu, 28 Mar 2024 12:24:48 +0100 Subject: [PATCH] x86/pat: fix W^X violation false-positives when running as Xen PV guest 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(). Fixes: 652c5bf380ad ("x86/mm: Refuse W^X violations") Reported-by: Jason Andryuk Signed-off-by: Juergen Gross --- 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, diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c index b59b09c2f284..e833183d1adb 100644 --- a/arch/x86/kernel/sev.c +++ b/arch/x86/kernel/sev.c @@ -515,12 +515,13 @@ 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", 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; 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); } /* @@ -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); diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c index cffe1157a90a..9131d80cff36 100644 --- a/arch/x86/virt/svm/sev.c +++ b/arch/x86/virt/svm/sev.c @@ -338,12 +338,13 @@ void snp_dump_hva_rmpentry(unsigned long hva) { unsigned long paddr; unsigned int level; + bool nx, rw; pgd_t *pgd; pte_t *pte; pgd = __va(read_cr3_pa()); pgd += pgd_index(hva); - pte = lookup_address_in_pgd(pgd, hva, &level); + pte = lookup_address_in_pgd(pgd, hva, &level, &nx, &rw); if (!pte) { pr_err("Can't dump RMP entry for HVA %lx: no PTE/PFN found\n", hva); -- 2.35.3