From da63b6b20077469bd6bd96e07991ce145fc4fbc4 Mon Sep 17 00:00:00 2001 From: Baoquan He Date: Thu, 27 Apr 2017 15:42:20 +0800 Subject: [PATCH] x86/KASLR: Fix kexec kernel boot crash when KASLR randomization fails Dave found that a kdump kernel with KASLR enabled will reset to the BIOS immediately if physical randomization failed to find a new position for the kernel. A kernel with the 'nokaslr' option works in this case. The reason is that KASLR will install a new page table for the identity mapping, while it missed building it for the original kernel location if KASLR physical randomization fails. This only happens in the kexec/kdump kernel, because the identity mapping has been built for kexec/kdump in the 1st kernel for the whole memory by calling init_pgtable(). Here if physical randomizaiton fails, it won't build the identity mapping for the original area of the kernel but change to a new page table '_pgtable'. Then the kernel will triple fault immediately caused by no identity mappings. The normal kernel won't see this bug, because it comes here via startup_32() and CR3 will be set to _pgtable already. In startup_32() the identity mapping is built for the 0~4G area. In KASLR we just append to the existing area instead of entirely overwriting it for on-demand identity mapping building. So the identity mapping for the original area of kernel is still there. To fix it we just switch to the new identity mapping page table when physical KASLR succeeds. Otherwise we keep the old page table unchanged just like "nokaslr" does. Signed-off-by: Baoquan He Signed-off-by: Dave Young Acked-by: Kees Cook Cc: Borislav Petkov Cc: Dave Jiang Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Garnier Cc: Thomas Gleixner Cc: Yinghai Lu Link: http://lkml.kernel.org/r/1493278940-5885-1-git-send-email-bhe@redhat.com Signed-off-by: Ingo Molnar --- arch/x86/boot/compressed/kaslr.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c index 6d9a546ec7ae..54c24f0a43d3 100644 --- a/arch/x86/boot/compressed/kaslr.c +++ b/arch/x86/boot/compressed/kaslr.c @@ -597,10 +597,17 @@ void choose_random_location(unsigned long input, add_identity_map(random_addr, output_size); *output = random_addr; } + + /* + * This loads the identity mapping page table. + * This should only be done if a new physical address + * is found for the kernel, otherwise we should keep + * the old page table to make it be like the "nokaslr" + * case. + */ + finalize_identity_maps(); } - /* This actually loads the identity pagetable on x86_64. */ - finalize_identity_maps(); /* Pick random virtual address starting from LOAD_PHYSICAL_ADDR. */ if (IS_ENABLED(CONFIG_X86_64)) -- 2.20.1