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 page_is_ram() to ensure correct memory bounds.

Fixes: 467d2782 ("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>
Cc: Ritesh Harjani <ritesh.list@gmail.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220531041446.3334259-1-vaibhav@linux.ibm.com