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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> |
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