[PATCH v2] kexec: fix out of the ELF headers buffer issue in syscall kexec_file_load()

From: Lee, Chun-Yi
Date: Mon Sep 28 2015 - 09:54:10 EST

This patch modified the code in fill_up_crash_elf_data by using
walk_system_ram_res instead of walk_system_ram_range to count the max
number of crash memory ranges. That's because the walk_system_ram_range
filters out small memory regions that are resided in the same page, but
walk_system_ram_res does not.

The oringial issue is page fault error that sometimes happened on big machines
when preparing ELF headers:

[ 305.291522] BUG: unable to handle kernel paging request at ffffc90613fc9000
[ 305.299621] IP: [<ffffffff8103d645>] prepare_elf64_ram_headers_callback+0x165/0x260
[ 305.308300] PGD e000032067 PUD 6dcbec54067 PMD 9dc9bdeb067 PTE 0
[ 305.315393] Oops: 0002 [#1] SMP
[ 305.420953] task: ffff8e1c01ced600 ti: ffff8e1c03ec2000 task.ti: ffff8e1c03ec2000
[ 305.429292] RIP: 0010:[<ffffffff8103d645>] [<ffffffff8103d645>] prepare_elf64_ra

After tracing prepare_elf64_headers and prepare_elf64_ram_headers_callback,
the code uses walk_system_ram_res to fill-in crash memory regions information
to program header, so it counts those small memory regions that are resided in
a page area. But, when kernel was using walk_system_ram_range in
fill_up_crash_elf_data to count the number of crash memory regions, it filters
out small regions. I printed those small memory regions, for example:

kexec: Get nr_ram ranges. vaddr=0xffff880077592258 paddr=0x77592258, sz=0xdc0

Base on the code in walk_system_ram_range, this memory region will be filtered

pfn = (0x77592258 + 0x1000 - 1) >> 12 = 0x77593
end_pfn = (0x77592258 + 0xfc0 -1 + 1) >> 12 = 0x77593
end_pfn - pfn = 0x77593 - 0x77593 = 0 <=== if (end_pfn > pfn) = FALSE

So, the max_nr_ranges that's counted by kernel doesn't include small memory
regions. That causes the page fault issue happened in later code path for
preparing EFL headers.

This issus doesn't easy to reproduce on small machines that don't have too
many CPU because the allocated page aligned ELF buffer has more free space
to cover those small memory regions' PT_LOAD headers.

To simplify the patch description, removed some things about CPU number to
avoid confusing patch reviewer.

Signed-off-by: Lee, Chun-Yi <jlee@xxxxxxxx>
arch/x86/kernel/crash.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index e068d66..ad273b3d 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -185,8 +185,7 @@ void native_machine_crash_shutdown(struct pt_regs *regs)

-static int get_nr_ram_ranges_callback(unsigned long start_pfn,
- unsigned long nr_pfn, void *arg)
+static int get_nr_ram_ranges_callback(u64 start, u64 end, void *arg)
int *nr_ranges = arg;

@@ -214,7 +213,7 @@ static void fill_up_crash_elf_data(struct crash_elf_data *ced,

ced->image = image;

- walk_system_ram_range(0, -1, &nr_ranges,
+ walk_system_ram_res(0, -1, &nr_ranges,

ced->max_nr_ranges = nr_ranges;

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