4.9-stable review patch. If anyone has any objections, please let me know.
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From: Baoquan He bhe@redhat.com
[ Upstream commit da63b6b20077469bd6bd96e07991ce145fc4fbc4 ]
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 bhe@redhat.com Signed-off-by: Dave Young dyoung@redhat.com Acked-by: Kees Cook keescook@chromium.org Cc: Borislav Petkov bp@suse.de Cc: Dave Jiang dave.jiang@intel.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Garnier thgarnie@google.com Cc: Thomas Gleixner tglx@linutronix.de Cc: Yinghai Lu yinghai@kernel.org Link: http://lkml.kernel.org/r/1493278940-5885-1-git-send-email-bhe@redhat.com Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/boot/compressed/kaslr.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)
--- a/arch/x86/boot/compressed/kaslr.c +++ b/arch/x86/boot/compressed/kaslr.c @@ -460,10 +460,17 @@ void choose_random_location(unsigned lon 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))