[PATCH] x86/kexec: Add a sanity check on previous kernel's ima kexec buffer

[Harshit Mogalapalli]

When the second-stage kernel is booted via kexec with a limiting command
line such as "mem=<size>", the physical range that contains the carried
over IMA measurement list may fall outside the truncated RAM leading to
a kernel panic.

    BUG: unable to handle page fault for address: ffff97793ff47000
    RIP: ima_restore_measurement_list+0xdc/0x45a
    #PF: error_code(0x0000) – not-present page

Other architectures already validate the range with page_is_ram(), as
done in commit: cbf9c4b9617b ("of: check previous kernel's
ima-kexec-buffer against memory bounds") do a similar check on x86.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: b69a2afd5afc ("x86/kexec: Carry forward IMA measurement log on kexec")
Reported-by: Paul Webb <paul.x.webb@xxxxxxxxxx>
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx>
---
Have tested the kexec for x86 kernel with IMA_KEXEC enabled and the
above patch works good. Paul initially reported this on 6.12 kernel but
I was able to reproduce this on 6.18, so I tried replicating how this
was fixed in drivers/of/kexec.c
---
 arch/x86/kernel/setup.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 1b2edd07a3e1..fcef197d180e 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -439,9 +439,23 @@ int __init ima_free_kexec_buffer(void)
 
 int __init ima_get_kexec_buffer(void **addr, size_t *size)
 {
+	unsigned long start_pfn, end_pfn;
+
 	if (!ima_kexec_buffer_size)
 		return -ENOENT;
 
+	/*
+	 * Calculate the PFNs for the buffer and ensure
+	 * they are with in addressable memory.
+	 */
+	start_pfn = PFN_DOWN(ima_kexec_buffer_phys);
+	end_pfn = PFN_DOWN(ima_kexec_buffer_phys + ima_kexec_buffer_size - 1);
+	if (!pfn_range_is_mapped(start_pfn, end_pfn)) {
+		pr_warn("IMA buffer at 0x%llx, size = 0x%zx beyond memory\n",
+			ima_kexec_buffer_phys, ima_kexec_buffer_size);
+		return -EINVAL;
+	}
+
 	*addr = __va(ima_kexec_buffer_phys);
 	*size = ima_kexec_buffer_size;
 
-- 
2.50.1