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, 15 insertions, 0 deletions

diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 787f9e385e6d..6cd645edcf35 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -25,6 +25,8 @@ struct page_change_data {
 	pgprot_t clear_mask;
 };
 
+bool rodata_full __ro_after_init = IS_ENABLED(CONFIG_RODATA_FULL_DEFAULT_ENABLED);
+
 static int change_page_range(pte_t *ptep, pgtable_t token, unsigned long addr,
 			void *data)
 {
@@ -64,6 +66,7 @@ static int change_memory_common(unsigned long addr, int numpages,
 	unsigned long size = PAGE_SIZE*numpages;
 	unsigned long end = start + size;
 	struct vm_struct *area;
+	int i;
 
 	if (!PAGE_ALIGNED(addr)) {
 		start &= PAGE_MASK;
@@ -94,6 +97,18 @@ static int change_memory_common(unsigned long addr, int numpages,
 		return 0;
 
 	/*
+	 * If we are manipulating read-only permissions, apply the same
+	 * change to the linear mapping of the pages that back this VM area.
+	 */
+	if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY ||
+			    pgprot_val(clear_mask) == PTE_RDONLY)) {
+		for (i = 0; i < area->nr_pages; i++) {
+			__change_memory_common((u64)page_address(area->pages[i]),
+					       PAGE_SIZE, set_mask, clear_mask);
+		}
+	}
+
+	/*
 	 * Get rid of potentially aliasing lazily unmapped vm areas that may
 	 * have permissions set that deviate from the ones we are setting here.
 	 */