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Thu, 10 Apr 2025 12:27:23 -0700 (PDT) Message-ID: Date: Thu, 10 Apr 2025 22:27:21 +0300 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] x86/sev: Disallow userspace access to BIOS region for SEV-SNP guests To: Dan Williams , Kees Cook Cc: Dave Hansen , Tom Lendacky , Naveen N Rao , linux-kernel@vger.kernel.org, x86@kernel.org, linux-coco@lists.linux.dev, Dave Hansen , Borislav Petkov , Vishal Annapurve , Kirill Shutemov , Kevin Loughlin , Michal Hocko References: <67eedc35be77d_464ec29462@dwillia2-xfh.jf.intel.com.notmuch> <1bc4c506-57ad-38aa-d56d-ed058f54708e@amd.com> <67f5b75c37143_71fe2949b@dwillia2-xfh.jf.intel.com.notmuch> <202504091038.5D9B68A@keescook> <67f6bee647aa5_1302d294f5@dwillia2-xfh.jf.intel.com.notmuch> <202504100931.DEC3D3B79@keescook> <67f819faa4e0b_71fe294e1@dwillia2-xfh.jf.intel.com.notmuch> From: Nikolay Borisov Content-Language: en-US In-Reply-To: <67f819faa4e0b_71fe294e1@dwillia2-xfh.jf.intel.com.notmuch> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 10.04.25 г. 22:20 ч., Dan Williams wrote: > Nikolay Borisov wrote: > [..] >>>> Can't we simply treat return value of 2 for range_is_allowed the same way as >>>> if 0 was returned in mmap_mem and simply fail the call with -EPERM? >>> >>> The historical concern was that EPERM would break old tools. I don't >>> have any current evidence either way, though. >> >> Right, but we are only about to return 2 in a TVM context, so chances of >> running old tools are slim to none. Also it's perfectly valid to have >> mmap fail for a number of reasons, so old tools should be equipped with >> handling it returning -EPERM, no ? > > In practice that is yet another return code since the caller does not > know why the "2" is being returned and it is not clear how safe it is to > now start denying mmap in the !TVM case. So, perhaps something like this: > What I meant by "returning 2" is returning 2 from the call to range_is_allowed in mmap_mem and handling this value inside mmap_mem, not returning 2 to user space :) In essence something along the lines of: diff --git a/drivers/char/mem.c b/drivers/char/mem.c index 169eed162a7f..8273066b6637 100644 --- a/drivers/char/mem.c +++ b/drivers/char/mem.c @@ -359,7 +359,8 @@ static int mmap_mem(struct file *file, struct vm_area_struct *vma) if (!private_mapping_ok(vma)) return -ENOSYS; - if (!range_is_allowed(vma->vm_pgoff, size)) + int ret = range_is_allowed(vma->vm_pgoff, size); + if (!ret || ret == 2) return -EPERM; if (!phys_mem_access_prot_allowed(file, vma->vm_pgoff, size, > enum devmem_policy { > DEVMEM_DENY, > DEVMEM_ALLOW, > DEVMEM_ZERO_RW, /* XXX: fix mmap_mem to install zeroes? */ > DEVMEM_ZERO_RW_DENY_MMAP, > }; > > The hope is that legacy tools are either fine with open() failures due > to the prevalance of lockdown, fine with read/write of zeroes to BIOS > data due to the prevalance of CONFIG_STRICT_DEVMEM, or otherwise would > not notice / break when mmap() starts failing for BIOS data in the TVM > case. The !TVM case continues with the current gap for mmap. > > Or, rip the bandaid and do this to see who screams: > > enum devmem_policy { > DEVMEM_DENY, > DEVMEM_ALLOW, > DEVMEM_ZERO_RW_DENY_MMAP, > }; >