A capture kernel reads the memory of the crashed kernel, and that memory
may well have uncorrectable errors. On powerpc a machine check in the
kernel is only recovered from if the instruction that took it has an
exception table entry (cf. mce_common_process_ue()). copy_oldmem_page()
copies with copy_to_iter(). For a read() of /proc/vmcore that is a user
copy, all of whose loads have exception table entries, so the machine
check turns into a short copy and read_from_oldmem() returns -EFAULT.
But since commit 4a22fd20379c ("vmcore: convert __read_vmcore to use an
iov_iter") /proc/vmcore can be spliced too, and splice(), sendfile()
and copy_file_range() copy into kernel pages with memcpy(), which has
no exception table entries. A machine check there panics the capture
kernel. The same goes for the reads of the ELF headers and notes that
vmcore does itself.
Copy with _copy_mc_to_iter() instead. PPC64 selects ARCH_HAS_COPY_MC
and there it uses copy_mc_generic() for kernel and user destinations
alike, which has the exception table entries. Everywhere else it is
_copy_to_iter().
Reported-by: Chris Mason <[email protected]>
Fixes: 4a22fd20379c ("vmcore: convert __read_vmcore to use an iov_iter")
Cc: [email protected]
Signed-off-by: Christian Brauner (Amutable) <[email protected]>
---
arch/powerpc/kernel/crash_dump.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/powerpc/kernel/crash_dump.c b/arch/powerpc/kernel/crash_dump.c
index 1543917bd3e1..ca7e019ae3e9 100644
--- a/arch/powerpc/kernel/crash_dump.c
+++ b/arch/powerpc/kernel/crash_dump.c
@@ -83,12 +83,12 @@ ssize_t copy_oldmem_page(struct iov_iter *iter, unsigned
long pfn,
if (memblock_is_region_memory(paddr, csize)) {
vaddr = __va(paddr);
- csize = copy_to_iter(vaddr + offset, csize, iter);
+ csize = _copy_mc_to_iter(vaddr + offset, csize, iter);
} else {
vaddr = ioremap_cache(paddr, PAGE_SIZE);
if (!vaddr)
return -ENOMEM;
- csize = copy_to_iter(vaddr + offset, csize, iter);
+ csize = _copy_mc_to_iter(vaddr + offset, csize, iter);
iounmap(vaddr);
}
--
2.53.0
