[PATCH AUTOSEL 5.8 028/132] x86/kaslr: Initialize mem_limit to the real maximum address

Sasha Levin
2020-10-27T00:29:21Z

From: Arvind Sankar <nivedita@alum.mit.edu>

[ Upstream commit 451286940d95778e83fa7f97006316d995b4c4a8 ]

On 64-bit, the kernel must be placed below MAXMEM (64TiB with 4-level
paging or 4PiB with 5-level paging). This is currently not enforced by
KASLR, which thus implicitly relies on physical memory being limited to
less than 64TiB.

On 32-bit, the limit is KERNEL_IMAGE_SIZE (512MiB). This is enforced by
special checks in __process_mem_region().

Initialize mem_limit to the maximum (depending on architecture), instead
of ULLONG_MAX, and make sure the command-line arguments can only
decrease it. This makes the enforcement explicit on 64-bit, and
eliminates the 32-bit specific checks to keep the kernel below 512M.

Check upfront to make sure the minimum address is below the limit before
doing any work.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200727230801.3468620-5-nivedita@alum.mit.edu
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/boot/compressed/kaslr.c | 41 +++++++++++++++++---------------
1 file changed, 22 insertions(+), 19 deletions(-)

diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
index d7408af557386..b59592273c329 100644
--- a/arch/x86/boot/compressed/kaslr.c
+++ b/arch/x86/boot/compressed/kaslr.c
@@ -94,8 +94,11 @@ static unsigned long get_boot_seed(void)
static bool memmap_too_large;

-/* Store memory limit specified by "mem=nn[KMG]" or "memmap=nn[KMG]" */
-static unsigned long long mem_limit = ULLONG_MAX;
+/*
+ * Store memory limit: MAXMEM on 64-bit and KERNEL_IMAGE_SIZE on 32-bit.
+ * It may be reduced by "mem=nn[KMG]" or "memmap=nn[KMG]" command line options.
+ */
+static unsigned long long mem_limit;

/* Number of immovable memory regions */
static int num_immovable_mem;
@@ -221,7 +224,7 @@ static void mem_avoid_memmap(enum parse_mode mode, char *str)

if (start == 0) {
/* Store the specified memory limit if size > 0 */
- if (size > 0)
+ if (size > 0 && size < mem_limit)
mem_limit = size;

continue;
@@ -309,7 +312,8 @@ static void handle_mem_options(void)
if (mem_size == 0)
goto out;

- mem_limit = mem_size;
+ if (mem_size < mem_limit)
+ mem_limit = mem_size;
} else if (!strcmp(param, "efi_fake_mem")) {
mem_avoid_memmap(PARSE_EFI, val);
}
@@ -321,7 +325,9 @@ static void handle_mem_options(void)
}

/*
- * In theory, KASLR can put the kernel anywhere in the range of [16M, 64T).
+ * In theory, KASLR can put the kernel anywhere in the range of [16M, MAXMEM)
+ * on 64-bit, and [16M, KERNEL_IMAGE_SIZE) on 32-bit.
+ *
* The mem_avoid array is used to store the ranges that need to be avoided
* when KASLR searches for an appropriate random address. We must avoid any
* regions that are unsafe to overlap with during decompression, and other
@@ -621,10 +627,6 @@ static void __process_mem_region(struct mem_vector *entry,
unsigned long start_orig, end;
struct mem_vector cur_entry;

- /* On 32-bit, ignore entries entirely above our maximum. */
- if (IS_ENABLED(CONFIG_X86_32) && entry->start >= KERNEL_IMAGE_SIZE)
- return;
-
/* Ignore entries entirely below our minimum. */
if (entry->start + entry->size < minimum)
return;
@@ -657,11 +659,6 @@ static void __process_mem_region(struct mem_vector *entry,
/* Reduce size by any delta from the original address. */
region.size -= region.start - start_orig;

- /* On 32-bit, reduce region size to fit within max size. */
- if (IS_ENABLED(CONFIG_X86_32) &&
- region.start + region.size > KERNEL_IMAGE_SIZE)
- region.size = KERNEL_IMAGE_SIZE - region.start;
-
/* Return if region can't contain decompressed kernel */
if (region.size < image_size)
return;
@@ -846,15 +843,16 @@ static void process_e820_entries(unsigned long minimum,
static unsigned long find_random_phys_addr(unsigned long minimum,
unsigned long image_size)
{
+ /* Bail out early if it's impossible to succeed. */
+ if (minimum + image_size > mem_limit)
+ return 0;
+
/* Check if we had too many memmaps. */
if (memmap_too_large) {
debug_putstr("Aborted memory entries scan (more than 4 memmap= args)!\n");
return 0;
}

- /* Make sure minimum is aligned. */
- minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);
-
if (process_efi_entries(minimum, image_size))
return slots_fetch_random();

@@ -867,8 +865,6 @@ static unsigned long find_random_virt_addr(unsigned long minimum,
{
unsigned long slots, random_addr;

- /* Make sure minimum is aligned. */
- minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);
/* Align image_size for easy slot calculations. */
image_size = ALIGN(image_size, CONFIG_PHYSICAL_ALIGN);

@@ -915,6 +911,11 @@ void choose_random_location(unsigned long input,
/* Prepare to add new identity pagetables on demand. */
initialize_identity_maps();

+ if (IS_ENABLED(CONFIG_X86_32))
+ mem_limit = KERNEL_IMAGE_SIZE;
+ else
+ mem_limit = MAXMEM;
+
/* Record the various known unsafe memory ranges. */
mem_avoid_init(input, input_size, *output);

@@ -924,6 +925,8 @@ void choose_random_location(unsigned long input,
* location:
*/
min_addr = min(*output, 512UL << 20);
+ /* Make sure minimum is aligned. */
+ min_addr = ALIGN(min_addr, CONFIG_PHYSICAL_ALIGN);

/* Walk available memory entries to find a random address. */
random_addr = find_random_phys_addr(min_addr, output_size);
--
2.25.1