[PATCH] x86/efistub: Omit physical KASLR when memory reservations exist

Ard Biesheuvel
2024-05-16T09:05:56Z

From: Ard Biesheuvel <ardb@kernel.org>

The legacy decompressor has elaborate logic to ensure that the
randomized physical placement of the decompressed kernel image does not
conflict with any memory reservations, including ones specified on the
command line using mem=, memmap=, efi_fake_mem= or hugepages=, which are
taken into account by the kernel proper at a later stage.

When booting in EFI mode, it is the firmware's job to ensure that the
chosen range does not conflict with any memory reservations that it
knows about, and this is trivially achieved by using the firmware's
memory allocation APIs.

That leaves reservations specified on the command line, though, which
the firmware knows nothing about, as these regions have no other special
significance to the platform. Since commit

a1b87d54f4e4 ("x86/efistub: Avoid legacy decompressor when doing EFI boot")

these reservations are not taken into account when randomizing the
physical placement, which may result in conflicts where the memory
cannot be reserved by the kernel proper because its own executable image
resides there.

To avoid having to duplicate or reuse the existing complicated logic,
disable physical KASLR entirely when such overrides are specified. These
are mostly diagnostic tools or niche features, and physical KASLR (as
opposed to virtual KASLR, which is much more important as it affects the
memory addresses observed by code executing in the kernel) is something
we can live without.

Closes: https://lkml.kernel.org/r/FA5F6719-8824-4B04-803E-82990E65E627%40akamai.com
Reported-by: Ben Chaney <bchaney@akamai.com>
Fixes: a1b87d54f4e4 ("x86/efistub: Avoid legacy decompressor when doing EFI boot")
Cc: <stable@vger.kernel.org> # v6.1+
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
drivers/firmware/efi/libstub/x86-stub.c | 28 +++++++++++++++++++++++--
1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
index d5a8182cf2e1..1983fd3bf392 100644
--- a/drivers/firmware/efi/libstub/x86-stub.c
+++ b/drivers/firmware/efi/libstub/x86-stub.c
@@ -776,6 +776,26 @@ static void error(char *str)
efi_warn("Decompression failed: %s\n", str);
}

+static const char *cmdline_memmap_override;
+
+static efi_status_t parse_options(const char *cmdline)
+{
+ static const char opts[][14] = {
+ "mem=", "memmap=", "efi_fake_mem=", "hugepages="
+ };
+
+ for (int i = 0; i < ARRAY_SIZE(opts); i++) {
+ const char *p = strstr(cmdline, opts[i]);
+
+ if (p == cmdline || (p > cmdline && isspace(p[-1]))) {
+ cmdline_memmap_override = opts[i];
+ break;
+ }
+ }
+
+ return efi_parse_options(cmdline);
+}
+
static efi_status_t efi_decompress_kernel(unsigned long *kernel_entry)
{
unsigned long virt_addr = LOAD_PHYSICAL_ADDR;
@@ -807,6 +827,10 @@ static efi_status_t efi_decompress_kernel(unsigned long *kernel_entry)
!memcmp(efistub_fw_vendor(), ami, sizeof(ami))) {
efi_debug("AMI firmware v2.0 or older detected - disabling physical KASLR\n");
seed[0] = 0;
+ } else if (cmdline_memmap_override) {
+ efi_info("%s detected on the kernel command line - disabling physical KASLR\n",
+ cmdline_memmap_override);
+ seed[0] = 0;
}

boot_params_ptr->hdr.loadflags |= KASLR_FLAG;
@@ -883,7 +907,7 @@ void __noreturn efi_stub_entry(efi_handle_t handle,
}

#ifdef CONFIG_CMDLINE_BOOL
- status = efi_parse_options(CONFIG_CMDLINE);
+ status = parse_options(CONFIG_CMDLINE);
if (status != EFI_SUCCESS) {
efi_err("Failed to parse options\n");
goto fail;
@@ -892,7 +916,7 @@ void __noreturn efi_stub_entry(efi_handle_t handle,
if (!IS_ENABLED(CONFIG_CMDLINE_OVERRIDE)) {
unsigned long cmdline_paddr = ((u64)hdr->cmd_line_ptr |
((u64)boot_params->ext_cmd_line_ptr << 32));
- status = efi_parse_options((char *)cmdline_paddr);
+ status = parse_options((char *)cmdline_paddr);
if (status != EFI_SUCCESS) {
efi_err("Failed to parse options\n");
goto fail;
--
2.45.0.rc1.225.g2a3ae87e7f-goog