arm64: mm: apply r/o permissions of VM areas to its linear alias as well

On arm64, we use block mappings and contiguous hints to map the linear
region, to minimize the TLB footprint. However, this means that the
entire region is mapped using read/write permissions, which we cannot
modify at page granularity without having to take intrusive measures to
prevent TLB conflicts.

This means the linear aliases of pages belonging to read-only mappings
(executable or otherwise) in the vmalloc region are also mapped read/write,
and could potentially be abused to modify things like module code, bpf JIT
code or other read-only data.

So let's fix this, by extending the set_memory_ro/rw routines to take
the linear alias into account. The consequence of enabling this is
that we can no longer use block mappings or contiguous hints, so in
cases where the TLB footprint of the linear region is a bottleneck,
performance may be affected.

Therefore, allow this feature to be runtime en/disabled, by setting
rodata=full (or 'on' to disable just this enhancement, or 'off' to
disable read-only mappings for code and r/o data entirely) on the
kernel command line. Also, allow the default value to be set via a
Kconfig option.

Tested-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 14 insertions, 2 deletions

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index d1d6601b385d..e1b2d58a311a 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -451,7 +451,7 @@ static void __init map_mem(pgd_t *pgdp)
 	struct memblock_region *reg;
 	int flags = 0;
 
-	if (debug_pagealloc_enabled())
+	if (rodata_full || debug_pagealloc_enabled())
 		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
 
 	/*
@@ -552,7 +552,19 @@ static void __init map_kernel_segment(pgd_t *pgdp, void *va_start, void *va_end,
 
 static int __init parse_rodata(char *arg)
 {
-	return strtobool(arg, &rodata_enabled);
+	int ret = strtobool(arg, &rodata_enabled);
+	if (!ret) {
+		rodata_full = false;
+		return 0;
+	}
+
+	/* permit 'full' in addition to boolean options */
+	if (strcmp(arg, "full"))
+		return -EINVAL;
+
+	rodata_enabled = true;
+	rodata_full = true;
+	return 0;
 }
 early_param("rodata", parse_rodata);