[PATCH] Put each per-cpu kdump ELF notes into a single page

[Petr Tesarik]

On architectures that use percpu-vm, the percpu region is not guaranteed
to be contiguous in physical space. However, fs/proc/vmcore.c expects
all ELF notes to be contiguous. If the ELF note happens to occupy
two non-adjacent physical pages, part of the note may be read from an
incorrect memory location by the kdump kernel, resulting in failure to
initialize /proc/vmcore (if the content of the following physical page,
incorrectly interpreted as an ELF note specifies a large number), wrong
register values or other apparent random memory corruption.

There is currently no mechanism to pass the virtual-to-physical mapping
of the percpu allocation to the kdump kernel. So, instead, I'm changing
the alignment of the ELF note buffer. Since sizeof(note_buf_t) is less
than PAGE_SIZE, aligning the buffer to the nearest higher power of 2
is enough to make sure that the buffer cannot cross a page boundary,
effectively ensuring that the whole buffer is contiguous in physical
space.

Signed-off-by: Petr Tesarik <ptesarik@xxxxxxx>
---
 kernel/kexec.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/kexec.c b/kernel/kexec.c
index 2bee072..cdab59d 100644
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -1610,7 +1610,8 @@ void crash_save_cpu(struct pt_regs *regs, int cpu)
 static int __init crash_notes_memory_init(void)
 {
 	/* Allocate memory for saving cpu registers. */
-	crash_notes = alloc_percpu(note_buf_t);
+	crash_notes = __alloc_percpu(sizeof(note_buf_t),
+				     roundup_pow_of_two(sizeof(note_buf_t)));
 	if (!crash_notes) {
 		pr_warn("Kexec: Memory allocation for saving cpu register states failed\n");
 		return -ENOMEM;
-- 
1.8.4.5
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