4.18-stable review patch. If anyone has any objections, please let me know.
------------------
From: Dave Hansen dave.hansen@linux.intel.com
commit c40a56a7818cfe735fc93a69e1875f8bba834483 upstream.
The kernel image is mapped into two places in the virtual address space (addresses without KASLR, of course):
1. The kernel direct map (0xffff880000000000) 2. The "high kernel map" (0xffffffff81000000)
We actually execute out of #2. If we get the address of a kernel symbol, it points to #2, but almost all physical-to-virtual translations point to
Parts of the "high kernel map" alias are mapped in the userspace page tables with the Global bit for performance reasons. The parts that we map to userspace do not (er, should not) have secrets. When PTI is enabled then the global bit is usually not set in the high mapping and just used to compensate for poor performance on systems which lack PCID.
This is fine, except that some areas in the kernel image that are adjacent to the non-secret-containing areas are unused holes. We free these holes back into the normal page allocator and reuse them as normal kernel memory. The memory will, of course, get *used* via the normal map, but the alias mapping is kept.
This otherwise unused alias mapping of the holes will, by default keep the Global bit, be mapped out to userspace, and be vulnerable to Meltdown.
Remove the alias mapping of these pages entirely. This is likely to fracture the 2M page mapping the kernel image near these areas, but this should affect a minority of the area.
The pageattr code changes *all* aliases mapping the physical pages that it operates on (by default). We only want to modify a single alias, so we need to tweak its behavior.
This unmapping behavior is currently dependent on PTI being in place. Going forward, we should at least consider doing this for all configurations. Having an extra read-write alias for memory is not exactly ideal for debugging things like random memory corruption and this does undercut features like DEBUG_PAGEALLOC or future work like eXclusive Page Frame Ownership (XPFO).
Before this patch:
current_kernel:---[ High Kernel Mapping ]--- current_kernel-0xffffffff80000000-0xffffffff81000000 16M pmd current_kernel-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd current_kernel-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte current_kernel-0xffffffff81e11000-0xffffffff82000000 1980K RW NX pte current_kernel-0xffffffff82000000-0xffffffff82600000 6M ro PSE GLB NX pmd current_kernel-0xffffffff82600000-0xffffffff82c00000 6M RW PSE NX pmd current_kernel-0xffffffff82c00000-0xffffffff82e00000 2M RW NX pte current_kernel-0xffffffff82e00000-0xffffffff83200000 4M RW PSE NX pmd current_kernel-0xffffffff83200000-0xffffffffa0000000 462M pmd
current_user:---[ High Kernel Mapping ]--- current_user-0xffffffff80000000-0xffffffff81000000 16M pmd current_user-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd current_user-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte current_user-0xffffffff81e11000-0xffffffff82000000 1980K RW NX pte current_user-0xffffffff82000000-0xffffffff82600000 6M ro PSE GLB NX pmd current_user-0xffffffff82600000-0xffffffffa0000000 474M pmd
After this patch:
current_kernel:---[ High Kernel Mapping ]--- current_kernel-0xffffffff80000000-0xffffffff81000000 16M pmd current_kernel-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd current_kernel-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte current_kernel-0xffffffff81e11000-0xffffffff82000000 1980K pte current_kernel-0xffffffff82000000-0xffffffff82400000 4M ro PSE GLB NX pmd current_kernel-0xffffffff82400000-0xffffffff82488000 544K ro NX pte current_kernel-0xffffffff82488000-0xffffffff82600000 1504K pte current_kernel-0xffffffff82600000-0xffffffff82c00000 6M RW PSE NX pmd current_kernel-0xffffffff82c00000-0xffffffff82c0d000 52K RW NX pte current_kernel-0xffffffff82c0d000-0xffffffff82dc0000 1740K pte
current_user:---[ High Kernel Mapping ]--- current_user-0xffffffff80000000-0xffffffff81000000 16M pmd current_user-0xffffffff81000000-0xffffffff81e00000 14M ro PSE GLB x pmd current_user-0xffffffff81e00000-0xffffffff81e11000 68K ro GLB x pte current_user-0xffffffff81e11000-0xffffffff82000000 1980K pte current_user-0xffffffff82000000-0xffffffff82400000 4M ro PSE GLB NX pmd current_user-0xffffffff82400000-0xffffffff82488000 544K ro NX pte current_user-0xffffffff82488000-0xffffffff82600000 1504K pte current_user-0xffffffff82600000-0xffffffffa0000000 474M pmd
[ tglx: Do not unmap on 32bit as there is only one mapping ]
Fixes: 0f561fce4d69 ("x86/pti: Enable global pages for shared areas") Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: Kees Cook keescook@google.com Cc: Andrea Arcangeli aarcange@redhat.com Cc: Juergen Gross jgross@suse.com Cc: Josh Poimboeuf jpoimboe@redhat.com Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Peter Zijlstra peterz@infradead.org Cc: Hugh Dickins hughd@google.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Borislav Petkov bp@alien8.de Cc: Andy Lutomirski luto@kernel.org Cc: Andi Kleen ak@linux.intel.com Cc: Joerg Roedel jroedel@suse.de Link: https://lkml.kernel.org/r/20180802225831.5F6A2BFC@viggo.jf.intel.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/include/asm/set_memory.h | 1 + arch/x86/mm/init.c | 26 ++++++++++++++++++++++++-- arch/x86/mm/pageattr.c | 13 +++++++++++++ 3 files changed, 38 insertions(+), 2 deletions(-)
--- a/arch/x86/include/asm/set_memory.h +++ b/arch/x86/include/asm/set_memory.h @@ -46,6 +46,7 @@ int set_memory_np(unsigned long addr, in int set_memory_4k(unsigned long addr, int numpages); int set_memory_encrypted(unsigned long addr, int numpages); int set_memory_decrypted(unsigned long addr, int numpages); +int set_memory_np_noalias(unsigned long addr, int numpages);
int set_memory_array_uc(unsigned long *addr, int addrinarray); int set_memory_array_wc(unsigned long *addr, int addrinarray); --- a/arch/x86/mm/init.c +++ b/arch/x86/mm/init.c @@ -782,8 +782,30 @@ void free_init_pages(char *what, unsigne */ void free_kernel_image_pages(void *begin, void *end) { - free_init_pages("unused kernel image", - (unsigned long)begin, (unsigned long)end); + unsigned long begin_ul = (unsigned long)begin; + unsigned long end_ul = (unsigned long)end; + unsigned long len_pages = (end_ul - begin_ul) >> PAGE_SHIFT; + + + free_init_pages("unused kernel image", begin_ul, end_ul); + + /* + * PTI maps some of the kernel into userspace. For performance, + * this includes some kernel areas that do not contain secrets. + * Those areas might be adjacent to the parts of the kernel image + * being freed, which may contain secrets. Remove the "high kernel + * image mapping" for these freed areas, ensuring they are not even + * potentially vulnerable to Meltdown regardless of the specific + * optimizations PTI is currently using. + * + * The "noalias" prevents unmapping the direct map alias which is + * needed to access the freed pages. + * + * This is only valid for 64bit kernels. 32bit has only one mapping + * which can't be treated in this way for obvious reasons. + */ + if (IS_ENABLED(CONFIG_X86_64) && cpu_feature_enabled(X86_FEATURE_PTI)) + set_memory_np_noalias(begin_ul, len_pages); }
void __ref free_initmem(void) --- a/arch/x86/mm/pageattr.c +++ b/arch/x86/mm/pageattr.c @@ -53,6 +53,7 @@ static DEFINE_SPINLOCK(cpa_lock); #define CPA_FLUSHTLB 1 #define CPA_ARRAY 2 #define CPA_PAGES_ARRAY 4 +#define CPA_NO_CHECK_ALIAS 8 /* Do not search for aliases */
#ifdef CONFIG_PROC_FS static unsigned long direct_pages_count[PG_LEVEL_NUM]; @@ -1486,6 +1487,9 @@ static int change_page_attr_set_clr(unsi
/* No alias checking for _NX bit modifications */ checkalias = (pgprot_val(mask_set) | pgprot_val(mask_clr)) != _PAGE_NX; + /* Has caller explicitly disabled alias checking? */ + if (in_flag & CPA_NO_CHECK_ALIAS) + checkalias = 0;
ret = __change_page_attr_set_clr(&cpa, checkalias);
@@ -1772,6 +1776,15 @@ int set_memory_np(unsigned long addr, in return change_page_attr_clear(&addr, numpages, __pgprot(_PAGE_PRESENT), 0); }
+int set_memory_np_noalias(unsigned long addr, int numpages) +{ + int cpa_flags = CPA_NO_CHECK_ALIAS; + + return change_page_attr_set_clr(&addr, numpages, __pgprot(0), + __pgprot(_PAGE_PRESENT), 0, + cpa_flags, NULL); +} + int set_memory_4k(unsigned long addr, int numpages) { return change_page_attr_set_clr(&addr, numpages, __pgprot(0),