On 4/11/2023 10:25 AM, Roberto Sassu wrote: > On Tue, 2023-04-11 at 09:42 -0700, Casey Schaufler wrote: >> On 4/11/2023 12:53 AM, Roberto Sassu wrote: >>> On Tue, 2023-04-11 at 03:22 -0400, Mimi Zohar wrote: >>>> Hi Roberto, >>>> >>>> Sorry for the delay in responding... >>> Hi Mimi >>> >>> no worries! >>> >>>> The patch description reads as though support for per LSM multiple >>>> xattrs is being added in this patch, though lsm_get_xattr_slot() only >>>> ever is incremented once for each LSM. To simplify review, it would be >>>> nice to mention that lsm_get_xattr_slot() would be called multiple >>>> times per LSM xattr. >>> Ok, I will mention it. >>> >>>> On Fri, 2023-03-31 at 14:32 +0200, Roberto Sassu wrote: >>>>> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx> >>>>> >>>>> Currently, security_inode_init_security() supports only one LSM providing >>>>> an xattr and EVM calculating the HMAC on that xattr, plus other inode >>>>> metadata. >>>>> >>>>> Allow all LSMs to provide one or multiple xattrs, by extending the security >>>>> blob reservation mechanism. Introduce the new lbs_xattr_count field of the >>>>> lsm_blob_sizes structure, so that each LSM can specify how many xattrs it >>>>> needs, and the LSM infrastructure knows how many xattr slots it should >>>>> allocate. >>>>> >>>>> Dynamically allocate the new_xattrs array to be populated by LSMs with the >>>>> inode_init_security hook, and pass it to the latter instead of the >>>>> name/value/len triple. Unify the !initxattrs and initxattrs case, simply >>>>> don't allocate the new_xattrs array in the former. >>>>> >>>>> Also, pass to the hook the number of xattrs filled by each LSM, so that >>>>> there are no gaps when the next LSM fills the array. Gaps might occur >>>>> because an LSM can legitimately request xattrs to the LSM infrastructure, >>>>> but not fill the reserved slots, if it was not initialized. >>>> The number of security xattrs permitted per LSM was discussed in the >>>> second paragraph. The first line of this paragraph needs to be updated >>>> to reflect the current number of security xattrs used, though that is >>>> more related to the new lsm_get_xattr_slot(). Or perhaps the entire >>>> paragraph is unnecessary, a remnant from >>>> security_check_compact_filled_xattrs(), and should be removed. >>> I would probably say in that paragraph that the number specified for >>> the lbs_xattr_count field determines how many times an LSM can call >>> lsm_get_xattr_slot(). >>> >>>>> Update the documentation of security_inode_init_security() to reflect the >>>>> changes, and fix the description of the xattr name, as it is not allocated >>>>> anymore. >>>>> >>>>> Finally, adapt both SELinux and Smack to use the new definition of the >>>>> inode_init_security hook, and to fill the reserved slots in the xattr >>>>> array. Introduce the lsm_get_xattr_slot() helper to retrieve an available >>>>> slot to fill, and to increment the number of filled slots. >>>>> >>>>> Move the xattr->name assignment after the xattr->value one, so that it is >>>>> done only in case of successful memory allocation. For Smack, also reserve >>>>> space for the other defined xattrs although they are not set yet in >>>>> smack_inode_init_security(). >>>> This Smack comment should be moved to the previous paragraph and even >>>> expanded explaining that lsm_get_xattr_slot() will be called for each >>>> additional security xattr. >>>> From previous Paul's and Casey's comments, Smack will have just two >>> xattrs, assuming that security.SMACK_TRASMUTE64 can be set in >>> smack_inode_init_security(). I will change this part accordingly once >>> Casey can have a look at the function. >> To be clear, Smack may use two xattrs from smack_inode_init_security(), >> SMACK64 and SMACK64_TRANSMUTE. SMACK64_TRANSMUTE is only set on directories. >> SMACK64_MMAP and SMACK64_EXEC can be set on files, but they have to be >> set explicitly. A file may have three xattrs, but only one from >> smack_inode_init_security(). >> >> I'm looking at the existing Smack function, and it includes checking for >> the transmute attribute. Your patch seems to have dropped this important >> behavior. That needs to be restored in any case. You can tell that you need >> to include the SMACK64_TRANSMUTE xattr if setting it is detected. > Uhm, I think it is simply omitted in the patch, not deleted. Oops. Right you are. > > I just sent a draft of the modifications required to set > SMACK64_TRANSMUTE in smack_inode_init_security(). Yup. I'll comment on that. > > Roberto > >>>>> Reported-by: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx> (EVM crash) >>>>> Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/ >>>>> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx> >>>>> --- >>>>> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h >>>>> index c2be66c669a..9eb9b686493 100644 >>>>> --- a/include/linux/lsm_hooks.h >>>>> +++ b/include/linux/lsm_hooks.h >>>>> @@ -28,6 +28,7 @@ >>>>> #include <linux/security.h> >>>>> #include <linux/init.h> >>>>> #include <linux/rculist.h> >>>>> +#include <linux/xattr.h> >>>>> >>>>> union security_list_options { >>>>> #define LSM_HOOK(RET, DEFAULT, NAME, ...) RET (*NAME)(__VA_ARGS__); >>>>> @@ -63,8 +64,27 @@ struct lsm_blob_sizes { >>>>> int lbs_ipc; >>>>> int lbs_msg_msg; >>>>> int lbs_task; >>>>> + int lbs_xattr_count; /* number of xattr slots in new_xattrs array */ >>>>> }; >>>>> >>>>> +/** >>>>> + * lsm_get_xattr_slot - Return the next available slot and increment the index >>>>> + * @xattrs: array storing LSM-provided xattrs >>>>> + * @xattr_count: number of already stored xattrs (updated) >>>>> + * >>>>> + * Retrieve the first available slot in the @xattrs array to fill with an xattr, >>>>> + * and increment @xattr_count. >>>>> + * >>>>> + * Return: The slot to fill in @xattrs if non-NULL, NULL otherwise. >>>>> + */ >>>>> +static inline struct xattr *lsm_get_xattr_slot(struct xattr *xattrs, >>>>> + int *xattr_count) >>>>> +{ >>>>> + if (unlikely(!xattrs)) >>>>> + return NULL; >>>>> + return xattrs + (*xattr_count)++; >>>> At some point, since lsm_get_xattr_slot() could be called multiple >>>> times from the same LSM, shouldn't there be some sort of bounds >>>> checking? >>>> From previous Paul's comments, I understood that he prefers to avoid >>> extra checks. It will be up to LSM developers to ensure that the API is >>> used correctly. >>> >>> Thanks >>> >>> Roberto >>>
