Messages in this thread | | | Date | Wed, 4 Jan 2023 08:40:06 -0600 | Subject | Re: [patch] crypto: ccp - Avoid page allocation failure warning for SEV_GET_ID2 | From | Tom Lendacky <> |
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On 1/3/23 17:18, David Rientjes wrote: > On Tue, 3 Jan 2023, Tom Lendacky wrote: > >> On 12/30/22 16:18, David Rientjes wrote: >>> For SEV_GET_ID2, the user provided length does not have a specified >>> limitation because the length of the ID may change in the future. The >>> kernel memory allocation, however, is implicitly limited to 4MB on x86 by >>> the page allocator, otherwise the kzalloc() will fail. >>> >>> When this happens, it is best not to spam the kernel log with the warning. >>> Simply fail the allocation and return ENOMEM to the user. >>> >>> Fixes: d6112ea0cb34 ("crypto: ccp - introduce SEV_GET_ID2 command") >>> Reported-by: Andy Nguyen <theflow@google.com> >>> Reported-by: Peter Gonda <pgonda@google.com> >>> Suggested-by: Herbert Xu <herbert@gondor.apana.org.au> >>> Signed-off-by: David Rientjes <rientjes@google.com> >>> --- >>> drivers/crypto/ccp/sev-dev.c | 9 ++++++++- >>> 1 file changed, 8 insertions(+), 1 deletion(-) >>> >>> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c >>> --- a/drivers/crypto/ccp/sev-dev.c >>> +++ b/drivers/crypto/ccp/sev-dev.c >>> @@ -881,7 +881,14 @@ static int sev_ioctl_do_get_id2(struct sev_issue_cmd >>> *argp) >>> input_address = (void __user *)input.address; >>> if (input.address && input.length) { >>> - id_blob = kzalloc(input.length, GFP_KERNEL); >>> + /* >>> + * The length of the ID shouldn't be assumed by software since >>> + * it may change in the future. The allocation size is >>> limited >>> + * to 1 << (PAGE_SHIFT + MAX_ORDER - 1) by the page allocator. >>> + * If the allocation fails, simply return ENOMEM rather than >>> + * warning in the kernel log. >>> + */ >>> + id_blob = kzalloc(input.length, GFP_KERNEL | __GFP_NOWARN); >> >> We could do this or we could have the driver invoke the API with a zero length >> to get the minimum buffer size needed for the call. The driver could then >> perform some validation checks comparing the supplied input.length to the >> returned length. If the driver can proceed, then if input.length is exactly 2x >> the minimum length, then kzalloc the 2 * minimum length, otherwise kzalloc the >> minimum length. This is a bit more complicated, though, compared to this fix. >> > > Thanks Tom. IIUC, this could be useful to identify situations where > input.length != min_length and input.length != min_length*2 and, in those > cases, return EINVAL? Or are there situations where this is actually a > valid input.length? > > I was assuming that the user was always doing its own SEV_GET_ID2 first to > determine the length and then use it for input.length, but perhaps that's > not the case and they are passing a bogus value.
Except that if the user was always doing that, then we wouldn't be worried about this case then. But, I think my method is overkill and the simple approach of this patch is the way to go.
Thanks, Tom
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