Messages in this thread | | | Date | Fri, 13 May 2022 20:47:50 +0000 | Subject | Re: [PATCH] KVM: SVM: Use kzalloc for sev ioctl interfaces to prevent kernel memory leak. | From | Ashish Kalra <> |
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Hello Peter,
On 5/13/22 20:09, Peter Gonda wrote: > On Fri, May 13, 2022 at 2:11 PM Ashish Kalra <ashkalra@amd.com> wrote: >> Hello Sean & Peter, >> >> On 5/13/22 14:49, Sean Christopherson wrote: >>> On Fri, May 13, 2022, Peter Gonda wrote: >>>> On Thu, May 12, 2022 at 4:23 PM Ashish Kalra <Ashish.Kalra@amd.com> wrote: >>>>> From: Ashish Kalra <ashish.kalra@amd.com> >>>>> >>>>> For some sev ioctl interfaces, the length parameter that is passed maybe >>>>> less than or equal to SEV_FW_BLOB_MAX_SIZE, but larger than the data >>>>> that PSP firmware returns. In this case, kmalloc will allocate memory >>>>> that is the size of the input rather than the size of the data. >>>>> Since PSP firmware doesn't fully overwrite the allocated buffer, these >>>>> sev ioctl interface may return uninitialized kernel slab memory. >>>>> >>>>> Reported-by: Andy Nguyen <theflow@google.com> >>>>> Suggested-by: David Rientjes <rientjes@google.com> >>>>> Suggested-by: Peter Gonda <pgonda@google.com> >>>>> Cc: kvm@vger.kernel.org >>>>> Cc: linux-kernel@vger.kernel.org >>>>> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com> >>>>> --- >>>>> arch/x86/kvm/svm/sev.c | 6 +++--- >>>>> 1 file changed, 3 insertions(+), 3 deletions(-) >>>>> >>>> Can we just update all the kmalloc()s that buffers get given to the >>>> PSP? For instance doesn't sev_send_update_data() have an issue? >>>> Reading the PSP spec it seems like a user can call this ioctl with a >>>> large hdr_len and the PSP will only fill out what's actually required >>>> like in these fixed up cases? This is assuming the PSP is written to >>>> spec (and just the current version). I'd rather have all of these >>>> instances updated. >> Yes, this function is also vulnerable as it allocates the return buffer >> using kmalloc() and copies back to user the buffer sized as per the user >> provided length (and not the FW returned length), so it surely needs fixup. >> >> I will update all these instances to use kzalloc() instead of kmalloc(). > Do we need the alloc_page() in __sev_dbg_encrypt_user() to have __GFP_ZERO too?
Actually this is used to allocate intermediate buffers to do copy_from_user, so it should be safe as size here is used for actual guest memory size to be encrypted.
Thanks, Ashish > > >>> Agreed, the kernel should explicitly initialize any copy_to_user() to source and >>> never rely on the PSP to fill the entire blob unless there's an ironclad guarantee >>> the entire struct/blob will be written. E.g. it's probably ok to skip zeroing >>> "data" in sev_ioctl_do_platform_status(), but even then it might be wortwhile as >>> defense-in-depth. >>> >>> Looking through other copy_to_user() calls: >>> >>> - "blob" in sev_ioctl_do_pek_csr() >>> - "id_blob" in sev_ioctl_do_get_id2() >>> - "pdh_blob" and "cert_blob" in sev_ioctl_do_pdh_export() >> These functions are part of the ccp driver and a fix for them has >> already been sent upstream to linux-crypto@vger.kernel.org and >> linux-kernel@vger.kernel.org: >> >> [PATCH] crypto: ccp - Use kzalloc for sev ioctl interfaces to prevent >> kernel memory leak >> >> Thanks, >> >> Ashish >> >>> The last one is probably fine since the copy length comes from the PSP, but it's >>> not like these ioctls are performance critical... >>> >>> /* If we query the length, FW responded with expected data. */ >>> input.cert_chain_len = data.cert_chain_len; >>> input.pdh_cert_len = data.pdh_cert_len;
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