On Fri, Jun 28, 2019 at 12:45:11PM -0700, Jaskaran Singh Khurana wrote: > > Hello Eric, > On Thu, 27 Jun 2019, Eric Biggers wrote: > > > On Wed, Jun 19, 2019 at 12:10:47PM -0700, Jaskaran Khurana wrote: > > > This patch set adds in-kernel pkcs7 signature checking for the roothash of > > > the dm-verity hash tree. > > > The verification is to support cases where the roothash is not secured by > > > Trusted Boot, UEFI Secureboot or similar technologies. > > > One of the use cases for this is for dm-verity volumes mounted after boot, > > > the root hash provided during the creation of the dm-verity volume has to > > > be secure and thus in-kernel validation implemented here will be used > > > before we trust the root hash and allow the block device to be created. > > > > > > Why we are doing validation in the Kernel? > > > > > > The reason is to still be secure in cases where the attacker is able to > > > compromise the user mode application in which case the user mode validation > > > could not have been trusted. > > > The root hash signature validation in the kernel along with existing > > > dm-verity implementation gives a higher level of confidence in the > > > executable code or the protected data. Before allowing the creation of > > > the device mapper block device the kernel code will check that the detached > > > pkcs7 signature passed to it validates the roothash and the signature is > > > trusted by builtin keys set at kernel creation. The kernel should be > > > secured using Verified boot, UEFI Secure Boot or similar technologies so we > > > can trust it. > > > > > > What about attacker mounting non dm-verity volumes to run executable > > > code? > > > > > > This verification can be used to have a security architecture where a LSM > > > can enforce this verification for all the volumes and by doing this it can > > > ensure that all executable code runs from signed and trusted dm-verity > > > volumes. > > > > > > Further patches will be posted that build on this and enforce this > > > verification based on policy for all the volumes on the system. > > > > > > > I don't understand your justification for this feature. > > > > If userspace has already been pwned severely enough for the attacker to be > > executing arbitrary code with CAP_SYS_ADMIN (which is what the device mapper > > ioctls need), what good are restrictions on loading more binaries from disk? > > > > Please explain your security model. > > > > - Eric > > > > In a datacenter like environment, this will protect the system from below > attacks: > > 1.Prevents attacker from deploying scripts that run arbitrary executables on the system. > 2.Prevents physically present malicious admin to run arbitrary code on the > machine. > > Regards, > Jaskaran So you are trying to protect against people who already have a root shell? Can't they just e.g. run /usr/bin/python and type in some Python code? Or run /usr/bin/curl and upload all your secret data to their server. - Eric
