From b56617bbcb473c25815d1bf475e326f84563b1de Mon Sep 17 00:00:00 2001 From: Alexander Graf Date: Sat, 21 Sep 2024 08:57:12 +0000 Subject: [PATCH] target/i386: Walk NPT in guest real mode When translating virtual to physical address with a guest CPU that supports nested paging (NPT), we need to perform every page table walk access indirectly through the NPT, which we correctly do. However, we treat real mode (no page table walk) special: In that case, we currently just skip any walks and translate VA -> PA. With NPT enabled, we also need to then perform NPT walk to do GVA -> GPA -> HPA which we fail to do so far. The net result of that is that TCG VMs with NPT enabled that execute real mode code (like SeaBIOS) end up with GPA==HPA mappings which means the guest accesses host code and data. This typically shows as failure to boot guests. This patch changes the page walk logic for NPT enabled guests so that we always perform a GVA -> GPA translation and then skip any logic that requires an actual PTE. That way, all remaining logic to walk the NPT stays and we successfully walk the NPT in real mode. Cc: qemu-stable@nongnu.org Fixes: fe441054bb3f0 ("target-i386: Add NPT support") Signed-off-by: Alexander Graf Reported-by: Eduard Vlad Reviewed-by: Richard Henderson Message-ID: <20240921085712.28902-1-graf@amazon.com> Signed-off-by: Richard Henderson --- target/i386/tcg/sysemu/excp_helper.c | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-) diff --git a/target/i386/tcg/sysemu/excp_helper.c b/target/i386/tcg/sysemu/excp_helper.c index 8fb05b1f53..8cb0d80177 100644 --- a/target/i386/tcg/sysemu/excp_helper.c +++ b/target/i386/tcg/sysemu/excp_helper.c @@ -150,6 +150,7 @@ static bool mmu_translate(CPUX86State *env, const TranslateParams *in, uint32_t pkr; int page_size; int error_code; + int prot; restart_all: rsvd_mask = ~MAKE_64BIT_MASK(0, env_archcpu(env)->phys_bits); @@ -298,7 +299,7 @@ static bool mmu_translate(CPUX86State *env, const TranslateParams *in, /* combine pde and pte nx, user and rw protections */ ptep &= pte ^ PG_NX_MASK; page_size = 4096; - } else { + } else if (pg_mode) { /* * Page table level 2 */ @@ -343,6 +344,15 @@ static bool mmu_translate(CPUX86State *env, const TranslateParams *in, ptep &= pte | PG_NX_MASK; page_size = 4096; rsvd_mask = 0; + } else { + /* + * No paging (real mode), let's tentatively resolve the address as 1:1 + * here, but conditionally still perform an NPT walk on it later. + */ + page_size = 0x40000000; + paddr = in->addr; + prot = PAGE_READ | PAGE_WRITE | PAGE_EXEC; + goto stage2; } do_check_protect: @@ -358,7 +368,7 @@ do_check_protect_pse36: goto do_fault_protect; } - int prot = 0; + prot = 0; if (!is_mmu_index_smap(in->mmu_idx) || !(ptep & PG_USER_MASK)) { prot |= PAGE_READ; if ((ptep & PG_RW_MASK) || !(is_user || (pg_mode & PG_MODE_WP))) { @@ -420,6 +430,7 @@ do_check_protect_pse36: /* merge offset within page */ paddr = (pte & PG_ADDRESS_MASK & ~(page_size - 1)) | (addr & (page_size - 1)); + stage2: /* * Note that NPT is walked (for both paging structures and final guest @@ -562,7 +573,7 @@ static bool get_physical_address(CPUX86State *env, vaddr addr, addr = (uint32_t)addr; } - if (likely(env->cr[0] & CR0_PG_MASK)) { + if (likely(env->cr[0] & CR0_PG_MASK || use_stage2)) { in.cr3 = env->cr[3]; in.mmu_idx = mmu_idx; in.ptw_idx = use_stage2 ? MMU_NESTED_IDX : MMU_PHYS_IDX;