linux-kernel - Re: [PATCH] crypto/ccp: Fix locking on alloc failure handling

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Message-ID: <7d4bde18-bbb7-4177-8577-b96c16f80d1d@amd.com>
Date: Tue, 24 Jun 2025 11:41:41 +1000
From: Alexey Kardashevskiy <aik@....com>
To: Tom Lendacky <thomas.lendacky@....com>, linux-crypto@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Ashish Kalra <ashish.kalra@....com>,
 John Allen <john.allen@....com>, Herbert Xu <herbert@...dor.apana.org.au>,
 "Borislav Petkov (AMD)" <bp@...en8.de>, Michael Roth <michael.roth@....com>
Subject: Re: [PATCH] crypto/ccp: Fix locking on alloc failure handling



On 21/6/25 05:20, Tom Lendacky wrote:
> On 6/17/25 04:43, Alexey Kardashevskiy wrote:
>> The __snp_alloc_firmware_pages() helper allocates pages in the firmware
>> state (alloc + rmpupdate). In case of failed rmpupdate, it tries
>> reclaiming pages with already changed state. This requires calling
>> the PSP firmware and since there is sev_cmd_mutex to guard such calls,
>> the helper takes a "locked" parameter so specify if the lock needs to
>> be held.
>>
>> Most calls happen from snp_alloc_firmware_page() which executes without
>> the lock. However
>>
>> commit 24512afa4336 ("crypto: ccp: Handle the legacy TMR allocation when SNP is enabled")
>>
>> switched sev_fw_alloc() from alloc_pages() (which does not call the PSP) to
>> __snp_alloc_firmware_pages() (which does) but did not account for the fact
>> that sev_fw_alloc() is called from __sev_platform_init_locked()
>> (via __sev_platform_init_handle_tmr()) and executes with the lock held.
>>
>> Add a "locked" parameter to __snp_alloc_firmware_pages().
>> Make sev_fw_alloc() use the new parameter to prevent potential deadlock in
>> rmp_mark_pages_firmware() if rmpupdate() failed.
> 
> Would it make sense to add the locked parameter to sev_fw_alloc(), too?

That would be another patch then, this one is a fix ;)

and I'd probably just ditch both snp_alloc_firmware_page() and sev_fw_alloc(), rename __snp_alloc_firmware_pages() to snp_alloc_firmware_page() and just use this one everywhere. Nobody needs page struct anyway, and the locking will be clear everywhere. Also do the same for snp_free_firmware_page().

It is just that snp_alloc_firmware_page() and snp_free_firmware_page() are EXPORT_SYMBOL_GPL,

> Right now there is only one caller of sev_fw_alloc(), but in the future,
> if some other path should call sev_fw_alloc() and that path doesn't have
> the lock, then we'll miss taking it.

I'd rather just ditch sev_fw_alloc(), does not look very useful. Thanks,



> Thanks,
> Tom
> 
>>
>> Fixes: 24512afa4336 ("crypto: ccp: Handle the legacy TMR allocation when SNP is enabled")
>> Signed-off-by: Alexey Kardashevskiy <aik@....com>
>> ---
>>   drivers/crypto/ccp/sev-dev.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
>> index 3451bada884e..16a11d5efe46 100644
>> --- a/drivers/crypto/ccp/sev-dev.c
>> +++ b/drivers/crypto/ccp/sev-dev.c
>> @@ -434,7 +434,7 @@ static int rmp_mark_pages_firmware(unsigned long paddr, unsigned int npages, boo
>>   	return rc;
>>   }
>>   
>> -static struct page *__snp_alloc_firmware_pages(gfp_t gfp_mask, int order)
>> +static struct page *__snp_alloc_firmware_pages(gfp_t gfp_mask, int order, bool locked)
>>   {
>>   	unsigned long npages = 1ul << order, paddr;
>>   	struct sev_device *sev;
>> @@ -453,7 +453,7 @@ static struct page *__snp_alloc_firmware_pages(gfp_t gfp_mask, int order)
>>   		return page;
>>   
>>   	paddr = __pa((unsigned long)page_address(page));
>> -	if (rmp_mark_pages_firmware(paddr, npages, false))
>> +	if (rmp_mark_pages_firmware(paddr, npages, locked))
>>   		return NULL;
>>   
>>   	return page;
>> @@ -463,7 +463,7 @@ void *snp_alloc_firmware_page(gfp_t gfp_mask)
>>   {
>>   	struct page *page;
>>   
>> -	page = __snp_alloc_firmware_pages(gfp_mask, 0);
>> +	page = __snp_alloc_firmware_pages(gfp_mask, 0, false);
>>   
>>   	return page ? page_address(page) : NULL;
>>   }
>> @@ -498,7 +498,7 @@ static void *sev_fw_alloc(unsigned long len)
>>   {
>>   	struct page *page;
>>   
>> -	page = __snp_alloc_firmware_pages(GFP_KERNEL, get_order(len));
>> +	page = __snp_alloc_firmware_pages(GFP_KERNEL, get_order(len), true);
>>   	if (!page)
>>   		return NULL;
>>   

-- 
Alexey