* Joerg Roedel joro@8bytes.org wrote:
From: Joerg Roedel jroedel@suse.de
When vmalloc_sync_all() iterates over the address space until FIX_ADDR_TOP it will sync the whole kernel address space starting from VMALLOC_START.
This is not a problem when the kernel address range is identical in all page-tables, but this is no longer the case when PTI is enabled on x86-32. In that case the per-process LDT is mapped in the kernel address range and vmalloc_sync_all() clears the LDT mapping for all processes.
To make LDT working again vmalloc_sync_all() must only iterate over the volatile parts of the kernel address range that are identical between all processes. This includes the VMALLOC and the PKMAP areas on x86-32.
The order of the ranges in the address space is:
VMALLOC -> PKMAP -> LDT -> CPU_ENTRY_AREA -> FIX_ADDR
So the right check in vmalloc_sync_all() is "address < LDT_BASE_ADDR" to make sure the VMALLOC and PKMAP areas are synchronized and the LDT mapping is not falsely overwritten. the CPU_ENTRY_AREA and the FIXMAP area are no longer synced as well, but these ranges are synchronized on page-table creation time and do not change during runtime.
Note that the last sentence is not really true, because various fixmap PTE entries and the CEA areas may change: ACPI uses a dynamic fixmap entry in ghes_map() and PTI uses dynamic PTEs as well, such as when mapping the debug store in alloc_bts_buffer(), etc.
What you wanted to say is probably that on 32-bit kernels with !SHARED_KERNEL_PMD page table layouts the init_mm.pgd is the 'reference kernel page table', which, whenever vmalloc pmds get removed, must be copied over into all page tables listed in pgd_list.
(The addition of vmalloc PMD and PTE entries is lazy processed, at fault time.)
The vmalloc_sync_all() also iterating over the LDT range is buggy, because for the LDT the mappings are *intentionally* and fundamentally different between processes, i.e. not synchronized.
Furthermore I'm not sure we need to iterate over the PKMAP range either: those are effectively permanent PMDs as well, and they are not part of the vmalloc.c lazy deallocation scheme in any case - they are handled entirely separately in mm/highmem.c et al.
The reason vmalloc_sync_all() doesn't wreck the pkmap range is really just accidental, because kmap() is a globally synchronized mapping concept as well - but it doesn't actually remove pmds.
Anyway, below is the patch modified to only iterate over the vmalloc ranges.
Note that VMALLOC_END is two guard pages short of the true end of the vmalloc area - this should not matter because vmalloc_sync_all() only looks down to the pmd depth, which is at least 2MB granular.
Note that this is *completely* untested - I might have wrecked PKMAP in my ignorance. Mind giving it a careful review and a test?
Thanks,
Ingo
===========================> Subject: x86/mm/32: Sync only to VMALLOC_END in vmalloc_sync_all() From: Joerg Roedel jroedel@suse.de Date: Tue, 26 Nov 2019 11:09:42 +0100
From: Joerg Roedel jroedel@suse.de
The job of vmalloc_sync_all() is to help the lazy freeing of vmalloc() ranges: before such vmap ranges are reused we make sure that they are unmapped from every task's page tables.
This is really easy on pagetable setups where the kernel page tables are shared between all tasks - this is the case on 32-bit kernels with SHARED_KERNEL_PMD = 1.
But on !SHARED_KERNEL_PMD 32-bit kernels this involves iterating over the pgd_list and clearing all pmd entries in the pgds that are cleared in the init_mm.pgd, which is the reference pagetable that the vmalloc() code uses.
In that context the current practice of vmalloc_sync_all() iterating until FIX_ADDR_TOP is buggy:
for (address = VMALLOC_START & PMD_MASK; address >= TASK_SIZE_MAX && address < FIXADDR_TOP; address += PMD_SIZE) { struct page *page;
Because iterating up to FIXADDR_TOP will involve a lot of non-vmalloc address ranges:
VMALLOC -> PKMAP -> LDT -> CPU_ENTRY_AREA -> FIX_ADDR
This is mostly harmless for the FIX_ADDR and CPU_ENTRY_AREA ranges that don't clear their pmds, but it's lethal for the LDT range, which relies on having different mappings in different processes, and 'synchronizing' them in the vmalloc sense corrupts those pagetable entries (clearing them).
This got particularly prominent with PTI, which turns SHARED_KERNEL_PMD off and makes this the dominant mapping mode on 32-bit.
To make LDT working again vmalloc_sync_all() must only iterate over the volatile parts of the kernel address range that are identical between all processes.
So the correct check in vmalloc_sync_all() is "address < VMALLOC_END" to make sure the VMALLOC areas are synchronized and the LDT mapping is not falsely overwritten.
The CPU_ENTRY_AREA and the FIXMAP area are no longer synced either, but this is not really a proplem since their PMDs get established during bootup and never change.
This change fixes the ldt_gdt selftest in my setup.
Reported-by: Borislav Petkov bp@suse.de Tested-by: Borislav Petkov bp@suse.de Signed-off-by: Joerg Roedel jroedel@suse.de Cc: stable@vger.kernel.org Cc: Andy Lutomirski luto@kernel.org Cc: Borislav Petkov bp@alien8.de Cc: Brian Gerst brgerst@gmail.com Cc: Dave Hansen dave.hansen@linux.intel.com Cc: H. Peter Anvin hpa@zytor.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Fixes: 7757d607c6b3: ("x86/pti: Allow CONFIG_PAGE_TABLE_ISOLATION for x86_32") Link: https://lkml.kernel.org/r/20191126100942.13059-1-joro@8bytes.org Signed-off-by: Ingo Molnar mingo@kernel.org --- arch/x86/mm/fault.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
Index: tip/arch/x86/mm/fault.c =================================================================== --- tip.orig/arch/x86/mm/fault.c +++ tip/arch/x86/mm/fault.c @@ -197,7 +197,7 @@ void vmalloc_sync_all(void) return;
for (address = VMALLOC_START & PMD_MASK; - address >= TASK_SIZE_MAX && address < FIXADDR_TOP; + address >= TASK_SIZE_MAX && address < VMALLOC_END; address += PMD_SIZE) { struct page *page;