If a relocatable kernel is loaded at a non-zero address and told not to
relocate to zero (kdump or RELOCATABLE_TEST), the mapping of the
interrupt code at zero is left with RWX permissions.
That is a security weakness, and leads to a warning at boot if
CONFIG_DEBUG_WX is enabled:
powerpc/mm: Found insecure W+X mapping at address
00000000056435bc/0xc000000000000000
WARNING: CPU: 1 PID: 1 at arch/powerpc/mm/ptdump/ptdump.c:193
note_page+0x484/0x4c0
CPU: 1 PID: 1 Comm: swapper/0 Not tainted 6.2.0-rc1-00001-g8ae8e98aea82-dirty
#175
Hardware name: IBM pSeries (emulated by qemu) POWER9 (raw) 0x4e1202 0xf000005
of:SLOF,git-dd0dca hv:linux,kvm pSeries
NIP: c0000000004a1c34 LR: c0000000004a1c30 CTR: 0000000000000000
REGS: c000000003503770 TRAP: 0700 Not tainted
(6.2.0-rc1-00001-g8ae8e98aea82-dirty)
MSR: 8000000002029033 <SF,VEC,EE,ME,IR,DR,RI,LE> CR: 24000220 XER: 00000000
CFAR: c000000000545a58 IRQMASK: 0
...
NIP note_page+0x484/0x4c0
LR note_page+0x480/0x4c0
Call Trace:
note_page+0x480/0x4c0 (unreliable)
ptdump_pmd_entry+0xc8/0x100
walk_pgd_range+0x618/0xab0
walk_page_range_novma+0x74/0xc0
ptdump_walk_pgd+0x98/0x170
ptdump_check_wx+0x94/0x100
mark_rodata_ro+0x30/0x70
kernel_init+0x78/0x1a0
ret_from_kernel_thread+0x5c/0x64
The fix has two parts. Firstly the pages from zero up to the end of
interrupts need to be marked read-only, so that they are left with R-X
permissions. Secondly the mapping logic needs to be taught to ensure
there is a page boundary at the end of the interrupt region, so that the
permission change only applies to the interrupt text, and not the region
following it.
Fixes: c55d7b5e6426 ("powerpc: Remove STRICT_KERNEL_RWX incompatibility with
RELOCATABLE")
Signed-off-by: Michael Ellerman <m...@ellerman.id.au>
---
arch/powerpc/mm/book3s64/radix_pgtable.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c
b/arch/powerpc/mm/book3s64/radix_pgtable.c
index 5a2384ed1727..26245aaf12b8 100644
--- a/arch/powerpc/mm/book3s64/radix_pgtable.c
+++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
@@ -234,6 +234,14 @@ void radix__mark_rodata_ro(void)
end = (unsigned long)__end_rodata;
radix__change_memory_range(start, end, _PAGE_WRITE);
+
+ for (start = PAGE_OFFSET; start < (unsigned long)_stext; start +=
PAGE_SIZE) {
+ end = start + PAGE_SIZE;
+ if (overlaps_interrupt_vector_text(start, end))
+ radix__change_memory_range(start, end, _PAGE_WRITE);
+ else
+ break;
+ }
}
void radix__mark_initmem_nx(void)
@@ -268,6 +276,11 @@ static unsigned long next_boundary(unsigned long addr,
unsigned long end)
// Relocatable kernel running at non-zero real address
if (stext_phys != 0) {
+ // The end of interrupts code at zero is a rodata boundary
+ unsigned long end_intr = __pa_symbol(__end_interrupts) -
stext_phys;
+ if (addr < end_intr)
+ return end_intr;
+
// Start of relocated kernel text is a rodata boundary
if (addr < stext_phys)
return stext_phys;
--
2.39.0
