> From: Stefan Berger [mailto:stefanb@xxxxxxxxxxxxx] > Sent: Wednesday, June 16, 2021 4:40 PM > On 6/16/21 9:22 AM, Roberto Sassu wrote: > > vfs_getxattr() differs from vfs_setxattr() in the way it obtains the xattr > > value. The former gives precedence to the LSMs, and if the LSMs don't > > provide a value, obtains it from the filesystem handler. The latter does > > the opposite, first invokes the filesystem handler, and if the filesystem > > does not support xattrs, passes the xattr value to the LSMs. > > > > The problem is that not necessarily the user gets the same xattr value that > > he set. For example, if he sets security.selinux with a value not > > terminated with '\0', he gets a value terminated with '\0' because SELinux > > adds it during the translation from xattr to internal representation > > (vfs_setxattr()) and from internal representation to xattr > > (vfs_getxattr()). > > > > Normally, this does not have an impact unless the integrity of xattrs is > > verified with EVM. The kernel and the user see different values due to the > > different functions used to obtain them: > > > > kernel (EVM): uses vfs_getxattr_alloc() which obtains the xattr value from > > the filesystem handler (raw value); > > > > user (ima-evm-utils): uses vfs_getxattr() which obtains the xattr value > > from the LSMs (normalized value). > > Maybe there should be another implementation similar to > vfs_getxattr_alloc() (or modify it) to behave like vfs_getxattr() but do > the memory allocation part so that the kernel sees what user space see > rather than modifying it with your patch so that user space now sees > something different than what it has been for years (previous > NUL-terminated SELinux xattr may not be NUL-terminated anymore)? I'm concerned that this would break HMACs/digital signatures calculated with raw values. An alternative would be to do the EVM verification twice if the first time didn't succeed (with vfs_getxattr_alloc() and with the new function that behaves like vfs_getxattr()). Roberto HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063 Managing Director: Li Peng, Li Jian, Shi Yanli > Stefan > > > > > > > > Given that the difference between the raw value and the normalized value > > should be just the additional '\0' not the rest of the content, this patch > > modifies vfs_getxattr() to compare the size of the xattr value obtained > > from the LSMs to the size of the raw xattr value. If there is a mismatch > > and the filesystem handler does not return an error, vfs_getxattr() returns > > the raw value. > > > > This patch should have a minimal impact on existing systems, because if the > > SELinux label is written with the appropriate tools such as setfiles or > > restorecon, there will not be a mismatch (because the raw value also has > > '\0'). > > > > In the case where the SELinux label is written directly with setfattr and > > without '\0', this patch helps to align EVM and ima-evm-utils in terms of > > result provided (due to the fact that they both verify the integrity of > > xattrs from raw values). > > > > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx> > > Tested-by: Mimi Zohar <zohar@xxxxxxxxxxxxx> > > --- > > fs/xattr.c | 15 +++++++++++++++ > > 1 file changed, 15 insertions(+) > > > > diff --git a/fs/xattr.c b/fs/xattr.c > > index 5c8c5175b385..412ec875aa07 100644 > > --- a/fs/xattr.c > > +++ b/fs/xattr.c > > @@ -420,12 +420,27 @@ vfs_getxattr(struct user_namespace *mnt_userns, > struct dentry *dentry, > > const char *suffix = name + XATTR_SECURITY_PREFIX_LEN; > > int ret = xattr_getsecurity(mnt_userns, inode, suffix, value, > > size); > > + int ret_raw; > > + > > /* > > * Only overwrite the return value if a security module > > * is actually active. > > */ > > if (ret == -EOPNOTSUPP) > > goto nolsm; > > + > > + if (ret < 0) > > + return ret; > > + > > + /* > > + * Read raw xattr if the size from the filesystem handler > > + * differs from that returned by xattr_getsecurity() and is > > + * equal or greater than zero. > > + */ > > + ret_raw = __vfs_getxattr(dentry, inode, name, NULL, 0); > > + if (ret_raw >= 0 && ret_raw != ret) > > + goto nolsm; > > + > > return ret; > > } > > nolsm:
