[PATCH v2] of: check previous kernel's ima-kexec-buffer against memory bounds

[Vaibhav Jain <vaibhav@linux.ibm.com>]

Presently ima_get_kexec_buffer() doesn't check if the previous kernel's
ima-kexec-buffer lies outside the addressable memory range. This can result
in a kernel panic if the new kernel is booted with 'mem=X' arg and the
ima-kexec-buffer was allocated beyond that range by the previous kernel.
The panic is usually of the form below:

$ sudo kexec --initrd initrd vmlinux --append='mem=16G'

<snip>
 BUG: Unable to handle kernel data access on read at 0xc000c01fff7f0000
 Faulting instruction address: 0xc000000000837974
 Oops: Kernel access of bad area, sig: 11 [#1]
<snip>
 NIP [c000000000837974] ima_restore_measurement_list+0x94/0x6c0
 LR [c00000000083b55c] ima_load_kexec_buffer+0xac/0x160
 Call Trace:
 [c00000000371fa80] [c00000000083b55c] ima_load_kexec_buffer+0xac/0x160
 [c00000000371fb00] [c0000000020512c4] ima_init+0x80/0x108
 [c00000000371fb70] [c0000000020514dc] init_ima+0x4c/0x120
 [c00000000371fbf0] [c000000000012240] do_one_initcall+0x60/0x2c0
 [c00000000371fcc0] [c000000002004ad0] kernel_init_freeable+0x344/0x3ec
 [c00000000371fda0] [c0000000000128a4] kernel_init+0x34/0x1b0
 [c00000000371fe10] [c00000000000ce64] ret_from_kernel_thread+0x5c/0x64
 Instruction dump:
 f92100b8 f92100c0 90e10090 910100a0 4182050c 282a0017 3bc00000 40810330
 7c0802a6 fb610198 7c9b2378 f80101d0 <a1240000> 2c090001 40820614 e9240010
 ---[ end trace 0000000000000000 ]---

Fix this issue by checking returned PFN range of previous kernel's
ima-kexec-buffer with pfn_valid to ensure correct memory bounds.

Fixes: 467d27824920 ("powerpc: ima: get the kexec buffer passed by the previous kernel")
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Prakhar Srivastava <prsriva@linux.microsoft.com>
Cc: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Cc: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>

---
Changelog
==========

v2:
* Instead of using memblock to determine the valid bounds use pfn_valid() to do
so since memblock may not be available late after the kernel init. [ Mpe ]
* Changed the patch prefix from 'powerpc' to 'of' [ Mpe ]
* Updated the 'Fixes' tag to point to correct commit that introduced this
function. [ Rob ]
* Fixed some whitespace/tab issues in the patch description [ Rob ]
* Added another check for checking ig 'tmp_size' for ima-kexec-buffer is > 0
---
 drivers/of/kexec.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/of/kexec.c b/drivers/of/kexec.c
index 8d374cc552be..879e984fe901 100644
--- a/drivers/of/kexec.c
+++ b/drivers/of/kexec.c
@@ -126,6 +126,7 @@ int ima_get_kexec_buffer(void **addr, size_t *size)
 {
 	int ret, len;
 	unsigned long tmp_addr;
+	unsigned int start_pfn, end_pfn;
 	size_t tmp_size;
 	const void *prop;
 
@@ -140,6 +141,22 @@ int ima_get_kexec_buffer(void **addr, size_t *size)
 	if (ret)
 		return ret;
 
+	/* Do some sanity on the returned size for the ima-kexec buffer */
+	if (!tmp_size)
+		return -ENOENT;
+
+	/*
+	 * Calculate the PFNs for the buffer and ensure
+	 * they are with in addressable memory.
+	 */
+	start_pfn = PHYS_PFN(tmp_addr);
+	end_pfn = PHYS_PFN(tmp_addr + tmp_size - 1);
+	if (!pfn_valid(start_pfn) || !pfn_valid(end_pfn)) {
+		pr_warn("IMA buffer at 0x%lx, size = 0x%zx beyond memory\n",
+			tmp_addr, tmp_size);
+		return -EINVAL;
+	}
+
 	*addr = __va(tmp_addr);
 	*size = tmp_size;
 
-- 
2.35.1