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

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

Hi Ritesh,
thanks for looking into this patch,

Ritesh Harjani <ritesh.list@gmail.com> writes:

> Just a minor nit which I noticed.
>
> On 22/05/24 11:20AM, Vaibhav Jain wrote:
>> 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;
>
> ^^^ Shouldn't this be unsigned long?
Thanks for catching this. Yes that should be 'unsigned long'. Will
resend the patch with this fixed.

>
> -ritesh
>
>>  	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
>>

-- 
Cheers
~ Vaibhav