arm64: map [_text, _stext) virtual address range non-executable+read-only

Since the referenced fixes commit, the kernel's .text section is only
mapped starting from _stext; the region [_text, _stext) is omitted. As a
result, other vmalloc/vmap allocations may use the virtual addresses
nominally in the range [_text, _stext). This address reuse confuses
multiple things:

1. crash_prepare_elf64_headers() sets up a segment in /proc/vmcore
   mapping the entire range [_text, _end) to
   [__pa_symbol(_text), __pa_symbol(_end)). Reading an address in
   [_text, _stext) from /proc/vmcore therefore gives the incorrect
   result.
2. Tools doing symbolization (either by reading /proc/kallsyms or based
   on the vmlinux ELF file) will incorrectly identify vmalloc/vmap
   allocations in [_text, _stext) as kernel symbols.

In practice, both of these issues affect the drgn debugger.
Specifically, there were cases where the vmap IRQ stacks for some CPUs
were allocated in [_text, _stext). As a result, drgn could not get the
stack trace for a crash in an IRQ handler because the core dump
contained invalid data for the IRQ stack address. The stack addresses
were also symbolized as being in the _text symbol.

Fix this by bringing back the mapping of [_text, _stext), but now make
it non-executable and read-only. This prevents other allocations from
using it while still achieving the original goal of not mapping
unpredictable data as executable. Other than the changed protection,
this is effectively a revert of the fixes commit.

Fixes: e2a073dde921 ("arm64: omit [_text, _stext) from permanent kernel mapping")
Cc: stable@vger.kernel.org
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 70c2ca813c18..524d34a0e921 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -279,7 +279,7 @@ void __init arm64_memblock_init(void)
 	 * Register the kernel text, kernel data, initrd, and initial
 	 * pagetables with memblock.
 	 */
-	memblock_reserve(__pa_symbol(_stext), _end - _stext);
+	memblock_reserve(__pa_symbol(_text), _end - _text);
 	if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && phys_initrd_size) {
 		/* the generic initrd code expects virtual addresses */
 		initrd_start = __phys_to_virt(phys_initrd_start);