[PATCH] efi/libstub: Allocate the INITRD table in ACPI reclaim memory

[Sebastien WENSKE]

Commit 6e62d1c6f2c7 ("efi: Wipe INITRD config table from memory after
consumption") makes the kernel clear the base and size of the
LINUX_EFI_INITRD_MEDIA_GUID configuration table once it has read them,
so that a kexec'd kernel no longer picks up a stale initrd.

However, the stub allocates that table from EFI_LOADER_DATA, which the
kernel that consumes it treats as ordinary RAM, while the configuration
table entry pointing at it survives kexec. A kexec'd kernel therefore
finds the same address in the configuration table, and clears 16 bytes
there again, in memory that may by then hold anything - including the
kexec'd kernel's own image, which kexec_file_load() is free to place
there.

Seen on x86_64 v7.2.10 under QEMU with Debian's OVMF 2025.02 Secure
Boot capable build (OVMF_CODE_4M.secboot.fd, no keys enrolled), the
initrd handed to the stub through the LINUX_EFI_INITRD_MEDIA_GUID
device path (systemd-stub UKI). A kexec reboot (kexec_file_load(),
bzImage64, with an initrd) loads the new kernel at 0x1ca00000; the
stale table is at 0x1d4bcf98, in the middle of tcp_parse_options() in
that image. The kexec'd kernel boots, then oopses on the first incoming
SYN, 16 bytes of its text zeroed:

  BUG: kernel NULL pointer dereference, address: 0000000000000002
  RIP: 0010:tcp_parse_options+0xe7/0x2f0
  Code: a5 4c 29 e1 89 c8 0f b6 4d 12 83 e0 3f 83 e1 c0 09 c1 88 4d 12 eb de 41 83 fe 04 75 d8 41 f6 44 24 0d 02 74 d0 45 85 d2 75 cb <0f> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 66 39 d0 89 d1
  Call Trace:
   <IRQ>
   tcp_conn_request+0x270/0x9d0
   tcp_rcv_state_process+0x147/0x1560
   tcp_v4_do_rcv+0xc1/0x260
   tcp_v4_rcv+0x110e/0x1190

A debug print added before the wipe shows what the kexec'd kernel reads
at the table's address - its own text:

  efi: INITRD table at 0x1d4bcf98:
       base 0xc274c085660241b7 size 0x8c0c1661455b70f

Whether a given kernel crashes depends on what happens to land at that
address: v7.2.9 clears 16 bytes elsewhere in its image without any
visible failure, and bisecting v7.2.9..v7.2.10 ends on an unrelated
commit that only shifts the image's layout.

Allocate the table from EFI_ACPI_RECLAIM_MEMORY instead, as the stub
already does for the RNG seed table so that it survives a kexec reboot.
The table then stays where it is across kexec, and the wipe always
lands on the table itself.

Fixes: 6e62d1c6f2c7 ("efi: Wipe INITRD config table from memory after consumption")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Sebastien WENSKE <sebastien@xxxxxxxxx>
---
Tested on x86_64 under QEMU (q35, KVM) with Debian's OVMF 2025.02
(OVMF_CODE_4M.secboot.fd and OVMF_CODE_4M.fd), booting a systemd-stub
UKI, then rebooting with kexec_file_load() and an initrd:

- v7.2.10: the oops above in 6 out of 6 kexec reboots under the
  secboot build (none under the plain one). With this patch: 2 out of
  2 come up, and two chained kexec'd kernels read the cleared table
  back (base 0x0 size 0x0), now in ACPI data memory.
- v7.3-rc7 without this patch: no visible failure, but the same debug
  print shows the kexec'd kernel reading its own image at the table's
  address - and so clearing 16 bytes of it:

    efi: INITRD table at 0x1d4bcf98:
         base 0xf413863894c40c4 size 0x415c74063c1246b6 - inside this
         kernel's image

  With this patch: kexec reboots (chained too) come up under both OVMF
  builds. It builds without warnings, W=1 included for libstub.

Not tested: other architectures, real hardware.

Reserving the table with efi_mem_reserve() in efi_config_parse_tables()
instead doesn't help on x86: efi_arch_mem_reserve() only handles
EFI_BOOT_SERVICES_DATA ("Failed to lookup EFI memory descriptor"), so
kexec still places the next kernel over the table. Note that a kernel
kexec'd from one without this patch still finds the table in loader
data memory.

 drivers/firmware/efi/libstub/efi-stub-helper.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
index f27f2e1f0..9293b8b31 100644
--- a/drivers/firmware/efi/libstub/efi-stub-helper.c
+++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
@@ -641,8 +641,14 @@ efi_status_t efi_load_initrd(efi_loaded_image_t *image,
 				     EFISTUB_EVT_INITRD) == EFI_SUCCESS)
 		efi_info("Measured initrd data into PCR 9\n");
 
-	status = efi_bs_call(allocate_pool, EFI_LOADER_DATA, sizeof(initrd),
-			     (void **)&tbl);
+	/*
+	 * Use EFI_ACPI_RECLAIM_MEMORY so the table survives a kexec reboot:
+	 * configuration tables do, and the kernel wipes this one once it has
+	 * consumed it - a kexec'd kernel wipes it again, at the same address,
+	 * which in EFI_LOADER_DATA memory may hold anything by then.
+	 */
+	status = efi_bs_call(allocate_pool, EFI_ACPI_RECLAIM_MEMORY,
+			     sizeof(initrd), (void **)&tbl);
 	if (status != EFI_SUCCESS)
 		goto free_initrd;
 

base-commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907
-- 
2.47.3