| Message ID | 20220901152632.970018-1-omosnace@redhat.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | fs: fix capable() call in simple_xattr_list() | expand |
On Thu, Sep 01, 2022 at 05:26:30PM +0200, Ondrej Mosnacek wrote: > The goal of these patches is to avoid calling capable() unconditionally > in simple_xattr_list(), which causes issues under SELinux (see > explanation in the second patch). > > The first patch tries to make this change safer by converting > simple_xattrs to use the RCU mechanism, so that capable() is not called > while the xattrs->lock is held. I didn't find evidence that this is an > issue in the current code, but it can't hurt to make that change > either way (and it was quite straightforward). Hey Ondrey, There's another patchset I'd like to see first which switches from a linked list to an rbtree to get rid of performance issues in this code that can be used to dos tmpfs in containers: https://lore.kernel.org/lkml/d73bd478-e373-f759-2acb-2777f6bba06f@openvz.org I don't think Vasily has time to continue with this so I'll just pick it up hopefully this or the week after LPC. Christian
On Mon, Sep 5, 2022 at 11:08 AM Christian Brauner <brauner@kernel.org> wrote: > On Thu, Sep 01, 2022 at 05:26:30PM +0200, Ondrej Mosnacek wrote: > > The goal of these patches is to avoid calling capable() unconditionally > > in simple_xattr_list(), which causes issues under SELinux (see > > explanation in the second patch). > > > > The first patch tries to make this change safer by converting > > simple_xattrs to use the RCU mechanism, so that capable() is not called > > while the xattrs->lock is held. I didn't find evidence that this is an > > issue in the current code, but it can't hurt to make that change > > either way (and it was quite straightforward). > > Hey Ondrey, > > There's another patchset I'd like to see first which switches from a > linked list to an rbtree to get rid of performance issues in this code > that can be used to dos tmpfs in containers: > > https://lore.kernel.org/lkml/d73bd478-e373-f759-2acb-2777f6bba06f@openvz.org > > I don't think Vasily has time to continue with this so I'll just pick it > up hopefully this or the week after LPC. Hm... does rbtree support lockless traversal? Because if not, that would make it impossible to fix the issue without calling capable() inside the critical section (or doing something complicated), AFAICT. Would rhashtable be a workable alternative to rbtree for this use case? Skimming <linux/rhashtable.h> it seems to support both lockless lookup and traversal using RCU. And according to its manpage, *listxattr(2) doesn't guarantee that the returned names are sorted. -- Ondrej Mosnacek Senior Software Engineer, Linux Security - SELinux kernel Red Hat, Inc.
On Mon, Sep 05, 2022 at 12:15:01PM +0200, Ondrej Mosnacek wrote: > On Mon, Sep 5, 2022 at 11:08 AM Christian Brauner <brauner@kernel.org> wrote: > > On Thu, Sep 01, 2022 at 05:26:30PM +0200, Ondrej Mosnacek wrote: > > > The goal of these patches is to avoid calling capable() unconditionally > > > in simple_xattr_list(), which causes issues under SELinux (see > > > explanation in the second patch). > > > > > > The first patch tries to make this change safer by converting > > > simple_xattrs to use the RCU mechanism, so that capable() is not called > > > while the xattrs->lock is held. I didn't find evidence that this is an > > > issue in the current code, but it can't hurt to make that change > > > either way (and it was quite straightforward). > > > > Hey Ondrey, > > > > There's another patchset I'd like to see first which switches from a > > linked list to an rbtree to get rid of performance issues in this code > > that can be used to dos tmpfs in containers: > > > > https://lore.kernel.org/lkml/d73bd478-e373-f759-2acb-2777f6bba06f@openvz.org > > > > I don't think Vasily has time to continue with this so I'll just pick it > > up hopefully this or the week after LPC. > > Hm... does rbtree support lockless traversal? Because if not, that The rfc that Vasily sent didn't allow for that at least. > would make it impossible to fix the issue without calling capable() > inside the critical section (or doing something complicated), AFAICT. > Would rhashtable be a workable alternative to rbtree for this use > case? Skimming <linux/rhashtable.h> it seems to support both lockless > lookup and traversal using RCU. And according to its manpage, > *listxattr(2) doesn't guarantee that the returned names are sorted. I've never used the rhashtable infrastructure in any meaningful way. All I can say from looking at current users that it looks like it could work well for us here: struct simple_xattr { struct rhlist_head rhlist_head; char *name; size_t size; char value[]; }; static const struct rhashtable_params simple_xattr_rhashtable = { .head_offset = offsetof(struct simple_xattr, rhlist_head), .key_offset = offsetof(struct simple_xattr, name), or sm like this.
On Mon, Sep 05, 2022 at 05:30:36PM +0200, Christian Brauner wrote: > On Mon, Sep 05, 2022 at 12:15:01PM +0200, Ondrej Mosnacek wrote: > > On Mon, Sep 5, 2022 at 11:08 AM Christian Brauner <brauner@kernel.org> wrote: > > > On Thu, Sep 01, 2022 at 05:26:30PM +0200, Ondrej Mosnacek wrote: > > > > The goal of these patches is to avoid calling capable() unconditionally > > > > in simple_xattr_list(), which causes issues under SELinux (see > > > > explanation in the second patch). > > > > > > > > The first patch tries to make this change safer by converting > > > > simple_xattrs to use the RCU mechanism, so that capable() is not called > > > > while the xattrs->lock is held. I didn't find evidence that this is an > > > > issue in the current code, but it can't hurt to make that change > > > > either way (and it was quite straightforward). > > > > > > Hey Ondrey, > > > > > > There's another patchset I'd like to see first which switches from a > > > linked list to an rbtree to get rid of performance issues in this code > > > that can be used to dos tmpfs in containers: > > > > > > https://lore.kernel.org/lkml/d73bd478-e373-f759-2acb-2777f6bba06f@openvz.org > > > > > > I don't think Vasily has time to continue with this so I'll just pick it > > > up hopefully this or the week after LPC. > > > > Hm... does rbtree support lockless traversal? Because if not, that > > The rfc that Vasily sent didn't allow for that at least. > > > would make it impossible to fix the issue without calling capable() > > inside the critical section (or doing something complicated), AFAICT. > > Would rhashtable be a workable alternative to rbtree for this use > > case? Skimming <linux/rhashtable.h> it seems to support both lockless > > lookup and traversal using RCU. And according to its manpage, > > *listxattr(2) doesn't guarantee that the returned names are sorted. > > I've never used the rhashtable infrastructure in any meaningful way. All > I can say from looking at current users that it looks like it could work > well for us here: > > struct simple_xattr { > struct rhlist_head rhlist_head; > char *name; > size_t size; > char value[]; > }; > > static const struct rhashtable_params simple_xattr_rhashtable = { > .head_offset = offsetof(struct simple_xattr, rhlist_head), > .key_offset = offsetof(struct simple_xattr, name), > > or sm like this. I have a patch in rough shape that converts struct simple_xattr to use an rhashtable: https://gitlab.com/brauner/linux/-/commits/fs.xattr.simple.rework/ Light testing, not a lot useful comments and no meaningful commit message as of yet but I'll get to that. Even though your issue is orthogonal to the performance issues I'm trying to fix I went back to your patch, Ondrej to apply it on top. But I think it has one problem. Afaict, by moving the capable() call from the top of the function into the actual traversal portion an unprivileged user can potentially learn whether a file has trusted.* xattrs set. At least if dmesg isn't restricted on the kernel. That may very well be the reason why the capable() call is on top. (Because the straightforward fix for this would be to just call capable() a single time if at least one trusted xattr is encountered and store the result. That's pretty easy to do by making turning the trusted variable into an int, setting it to -1, and only if it's -1 and a trusted xattr has been found call capable() and store the result.) One option to fix all of that is to switch simple_xattr_list() to use ns_capable_noaudit(&init_user_ns, CAP_SYS_ADMIN) which doesn't generate an audit event. I think this is even the correct thing to do as listing xattrs isn't a targeted operation. IOW, if the the user had used getxattr() to request a trusted.* xattr then logging a denial makes sense as the user explicitly wanted to retrieve a trusted.* xattr. But if the user just requested to list all xattrs then silently skipping trusted without logging an explicit denial xattrs makes sense. Does that sound acceptable?
On Wed, Nov 02, 2022 at 07:24:51PM +0100, Christian Brauner wrote: > On Mon, Sep 05, 2022 at 05:30:36PM +0200, Christian Brauner wrote: > > On Mon, Sep 05, 2022 at 12:15:01PM +0200, Ondrej Mosnacek wrote: > > > On Mon, Sep 5, 2022 at 11:08 AM Christian Brauner <brauner@kernel.org> wrote: > > > > On Thu, Sep 01, 2022 at 05:26:30PM +0200, Ondrej Mosnacek wrote: > > > > > The goal of these patches is to avoid calling capable() unconditionally > > > > > in simple_xattr_list(), which causes issues under SELinux (see > > > > > explanation in the second patch). > > > > > > > > > > The first patch tries to make this change safer by converting > > > > > simple_xattrs to use the RCU mechanism, so that capable() is not called > > > > > while the xattrs->lock is held. I didn't find evidence that this is an > > > > > issue in the current code, but it can't hurt to make that change > > > > > either way (and it was quite straightforward). > > > > > > > > Hey Ondrey, > > > > > > > > There's another patchset I'd like to see first which switches from a > > > > linked list to an rbtree to get rid of performance issues in this code > > > > that can be used to dos tmpfs in containers: > > > > > > > > https://lore.kernel.org/lkml/d73bd478-e373-f759-2acb-2777f6bba06f@openvz.org > > > > > > > > I don't think Vasily has time to continue with this so I'll just pick it > > > > up hopefully this or the week after LPC. > > > > > > Hm... does rbtree support lockless traversal? Because if not, that > > > > The rfc that Vasily sent didn't allow for that at least. > > > > > would make it impossible to fix the issue without calling capable() > > > inside the critical section (or doing something complicated), AFAICT. > > > Would rhashtable be a workable alternative to rbtree for this use > > > case? Skimming <linux/rhashtable.h> it seems to support both lockless > > > lookup and traversal using RCU. And according to its manpage, > > > *listxattr(2) doesn't guarantee that the returned names are sorted. > > > > I've never used the rhashtable infrastructure in any meaningful way. All > > I can say from looking at current users that it looks like it could work > > well for us here: > > > > struct simple_xattr { > > struct rhlist_head rhlist_head; > > char *name; > > size_t size; > > char value[]; > > }; > > > > static const struct rhashtable_params simple_xattr_rhashtable = { > > .head_offset = offsetof(struct simple_xattr, rhlist_head), > > .key_offset = offsetof(struct simple_xattr, name), > > > > or sm like this. > > I have a patch in rough shape that converts struct simple_xattr to use > an rhashtable: > > https://gitlab.com/brauner/linux/-/commits/fs.xattr.simple.rework/ > > Light testing, not a lot useful comments and no meaningful commit > message as of yet but I'll get to that. > > Even though your issue is orthogonal to the performance issues I'm > trying to fix I went back to your patch, Ondrej to apply it on top. > But I think it has one problem. > > Afaict, by moving the capable() call from the top of the function into > the actual traversal portion an unprivileged user can potentially learn > whether a file has trusted.* xattrs set. At least if dmesg isn't > restricted on the kernel. That may very well be the reason why the > capable() call is on top. > (Because the straightforward fix for this would be to just call > capable() a single time if at least one trusted xattr is encountered and > store the result. That's pretty easy to do by making turning the trusted > variable into an int, setting it to -1, and only if it's -1 and a > trusted xattr has been found call capable() and store the result.) > > One option to fix all of that is to switch simple_xattr_list() to use > > ns_capable_noaudit(&init_user_ns, CAP_SYS_ADMIN) > > which doesn't generate an audit event. > > I think this is even the correct thing to do as listing xattrs isn't a > targeted operation. IOW, if the the user had used getxattr() to request > a trusted.* xattr then logging a denial makes sense as the user > explicitly wanted to retrieve a trusted.* xattr. But if the user just > requested to list all xattrs then silently skipping trusted without > logging an explicit denial xattrs makes sense. > > Does that sound acceptable? Agreed, auditing that seems like unwanted noise.
On Wed, Nov 2, 2022 at 7:25 PM Christian Brauner <brauner@kernel.org> wrote: > On Mon, Sep 05, 2022 at 05:30:36PM +0200, Christian Brauner wrote: > > On Mon, Sep 05, 2022 at 12:15:01PM +0200, Ondrej Mosnacek wrote: > > > On Mon, Sep 5, 2022 at 11:08 AM Christian Brauner <brauner@kernel.org> wrote: > > > > On Thu, Sep 01, 2022 at 05:26:30PM +0200, Ondrej Mosnacek wrote: > > > > > The goal of these patches is to avoid calling capable() unconditionally > > > > > in simple_xattr_list(), which causes issues under SELinux (see > > > > > explanation in the second patch). > > > > > > > > > > The first patch tries to make this change safer by converting > > > > > simple_xattrs to use the RCU mechanism, so that capable() is not called > > > > > while the xattrs->lock is held. I didn't find evidence that this is an > > > > > issue in the current code, but it can't hurt to make that change > > > > > either way (and it was quite straightforward). > > > > > > > > Hey Ondrey, > > > > > > > > There's another patchset I'd like to see first which switches from a > > > > linked list to an rbtree to get rid of performance issues in this code > > > > that can be used to dos tmpfs in containers: > > > > > > > > https://lore.kernel.org/lkml/d73bd478-e373-f759-2acb-2777f6bba06f@openvz.org > > > > > > > > I don't think Vasily has time to continue with this so I'll just pick it > > > > up hopefully this or the week after LPC. > > > > > > Hm... does rbtree support lockless traversal? Because if not, that > > > > The rfc that Vasily sent didn't allow for that at least. > > > > > would make it impossible to fix the issue without calling capable() > > > inside the critical section (or doing something complicated), AFAICT. > > > Would rhashtable be a workable alternative to rbtree for this use > > > case? Skimming <linux/rhashtable.h> it seems to support both lockless > > > lookup and traversal using RCU. And according to its manpage, > > > *listxattr(2) doesn't guarantee that the returned names are sorted. > > > > I've never used the rhashtable infrastructure in any meaningful way. All > > I can say from looking at current users that it looks like it could work > > well for us here: > > > > struct simple_xattr { > > struct rhlist_head rhlist_head; > > char *name; > > size_t size; > > char value[]; > > }; > > > > static const struct rhashtable_params simple_xattr_rhashtable = { > > .head_offset = offsetof(struct simple_xattr, rhlist_head), > > .key_offset = offsetof(struct simple_xattr, name), > > > > or sm like this. > > I have a patch in rough shape that converts struct simple_xattr to use > an rhashtable: > > https://gitlab.com/brauner/linux/-/commits/fs.xattr.simple.rework/ > > Light testing, not a lot useful comments and no meaningful commit > message as of yet but I'll get to that. Looks mostly good at first glance. I left comments for some minor stuff I noticed. > Even though your issue is orthogonal to the performance issues I'm > trying to fix I went back to your patch, Ondrej to apply it on top. > But I think it has one problem. > > Afaict, by moving the capable() call from the top of the function into > the actual traversal portion an unprivileged user can potentially learn > whether a file has trusted.* xattrs set. At least if dmesg isn't > restricted on the kernel. That may very well be the reason why the > capable() call is on top. Technically it would be possible, for example with SELinux if the audit daemon is dead. Not a likely situation, but I agree it's better to be safe. > (Because the straightforward fix for this would be to just call > capable() a single time if at least one trusted xattr is encountered and > store the result. That's pretty easy to do by making turning the trusted > variable into an int, setting it to -1, and only if it's -1 and a > trusted xattr has been found call capable() and store the result.) That would also run into the conundrum of holding a lock while (potentially) calling into the LSM subsystem. And would it even fix the information leak? Unless I'm missing something it would only prevent a leak of the trusted xattr count, but not the presence of any trusted xattr. > One option to fix all of that is to switch simple_xattr_list() to use > > ns_capable_noaudit(&init_user_ns, CAP_SYS_ADMIN) > > which doesn't generate an audit event. > > I think this is even the correct thing to do as listing xattrs isn't a > targeted operation. IOW, if the the user had used getxattr() to request > a trusted.* xattr then logging a denial makes sense as the user > explicitly wanted to retrieve a trusted.* xattr. But if the user just > requested to list all xattrs then silently skipping trusted without > logging an explicit denial xattrs makes sense. > > Does that sound acceptable? Yes, I can't see any reason why that wouldn't be the best solution. Why haven't I thought of that? :) I guess you will want to submit a patch for it along with your rhashtable patch to avoid a conflict? Or would you like me to submit it separately? -- Ondrej Mosnacek Senior Software Engineer, Linux Security - SELinux kernel Red Hat, Inc.
On Thu, Nov 03, 2022 at 10:04:25AM +0100, Ondrej Mosnacek wrote: > On Wed, Nov 2, 2022 at 7:25 PM Christian Brauner <brauner@kernel.org> wrote: > > On Mon, Sep 05, 2022 at 05:30:36PM +0200, Christian Brauner wrote: > > > On Mon, Sep 05, 2022 at 12:15:01PM +0200, Ondrej Mosnacek wrote: > > > > On Mon, Sep 5, 2022 at 11:08 AM Christian Brauner <brauner@kernel.org> wrote: > > > > > On Thu, Sep 01, 2022 at 05:26:30PM +0200, Ondrej Mosnacek wrote: > > > > > > The goal of these patches is to avoid calling capable() unconditionally > > > > > > in simple_xattr_list(), which causes issues under SELinux (see > > > > > > explanation in the second patch). > > > > > > > > > > > > The first patch tries to make this change safer by converting > > > > > > simple_xattrs to use the RCU mechanism, so that capable() is not called > > > > > > while the xattrs->lock is held. I didn't find evidence that this is an > > > > > > issue in the current code, but it can't hurt to make that change > > > > > > either way (and it was quite straightforward). > > > > > > > > > > Hey Ondrey, > > > > > > > > > > There's another patchset I'd like to see first which switches from a > > > > > linked list to an rbtree to get rid of performance issues in this code > > > > > that can be used to dos tmpfs in containers: > > > > > > > > > > https://lore.kernel.org/lkml/d73bd478-e373-f759-2acb-2777f6bba06f@openvz.org > > > > > > > > > > I don't think Vasily has time to continue with this so I'll just pick it > > > > > up hopefully this or the week after LPC. > > > > > > > > Hm... does rbtree support lockless traversal? Because if not, that > > > > > > The rfc that Vasily sent didn't allow for that at least. > > > > > > > would make it impossible to fix the issue without calling capable() > > > > inside the critical section (or doing something complicated), AFAICT. > > > > Would rhashtable be a workable alternative to rbtree for this use > > > > case? Skimming <linux/rhashtable.h> it seems to support both lockless > > > > lookup and traversal using RCU. And according to its manpage, > > > > *listxattr(2) doesn't guarantee that the returned names are sorted. > > > > > > I've never used the rhashtable infrastructure in any meaningful way. All > > > I can say from looking at current users that it looks like it could work > > > well for us here: > > > > > > struct simple_xattr { > > > struct rhlist_head rhlist_head; > > > char *name; > > > size_t size; > > > char value[]; > > > }; > > > > > > static const struct rhashtable_params simple_xattr_rhashtable = { > > > .head_offset = offsetof(struct simple_xattr, rhlist_head), > > > .key_offset = offsetof(struct simple_xattr, name), > > > > > > or sm like this. > > > > I have a patch in rough shape that converts struct simple_xattr to use > > an rhashtable: > > > > https://gitlab.com/brauner/linux/-/commits/fs.xattr.simple.rework/ > > > > Light testing, not a lot useful comments and no meaningful commit > > message as of yet but I'll get to that. > > Looks mostly good at first glance. I left comments for some minor > stuff I noticed. > > > Even though your issue is orthogonal to the performance issues I'm > > trying to fix I went back to your patch, Ondrej to apply it on top. > > But I think it has one problem. > > > > Afaict, by moving the capable() call from the top of the function into > > the actual traversal portion an unprivileged user can potentially learn > > whether a file has trusted.* xattrs set. At least if dmesg isn't > > restricted on the kernel. That may very well be the reason why the > > capable() call is on top. > > Technically it would be possible, for example with SELinux if the > audit daemon is dead. Not a likely situation, but I agree it's better > to be safe. > > > (Because the straightforward fix for this would be to just call > > capable() a single time if at least one trusted xattr is encountered and > > store the result. That's pretty easy to do by making turning the trusted > > variable into an int, setting it to -1, and only if it's -1 and a > > trusted xattr has been found call capable() and store the result.) > > That would also run into the conundrum of holding a lock while > (potentially) calling into the LSM subsystem. And would it even fix > the information leak? Unless I'm missing something it would only > prevent a leak of the trusted xattr count, but not the presence of any > trusted xattr. No it wouldn't. I just meant this to illustrate that with your patch we could've made it so that capable() would've only been called once. > > > One option to fix all of that is to switch simple_xattr_list() to use > > > > ns_capable_noaudit(&init_user_ns, CAP_SYS_ADMIN) > > > > which doesn't generate an audit event. > > > > I think this is even the correct thing to do as listing xattrs isn't a > > targeted operation. IOW, if the the user had used getxattr() to request > > a trusted.* xattr then logging a denial makes sense as the user > > explicitly wanted to retrieve a trusted.* xattr. But if the user just > > requested to list all xattrs then silently skipping trusted without > > logging an explicit denial xattrs makes sense. > > > > Does that sound acceptable? > > Yes, I can't see any reason why that wouldn't be the best solution. > Why haven't I thought of that? :) > > I guess you will want to submit a patch for it along with your > rhashtable patch to avoid a conflict? Or would you like me to submit > it separately? I think you can send a patch for this separately as we don't need to massage the data structure for this. I think we can reasonably give this a Fixes: 38f38657444d ("xattr: extract simple_xattr code from tmpfs") # no backport But note the "# no backport" as imho it isn't worth backporting this to older kernels unless that's really desirable.
On Thu, Nov 3, 2022 at 10:12 AM Christian Brauner <brauner@kernel.org> wrote: > On Thu, Nov 03, 2022 at 10:04:25AM +0100, Ondrej Mosnacek wrote: > > On Wed, Nov 2, 2022 at 7:25 PM Christian Brauner <brauner@kernel.org> wrote: > > > On Mon, Sep 05, 2022 at 05:30:36PM +0200, Christian Brauner wrote: > > > > On Mon, Sep 05, 2022 at 12:15:01PM +0200, Ondrej Mosnacek wrote: > > > > > On Mon, Sep 5, 2022 at 11:08 AM Christian Brauner <brauner@kernel.org> wrote: > > > > > > On Thu, Sep 01, 2022 at 05:26:30PM +0200, Ondrej Mosnacek wrote: > > > > > > > The goal of these patches is to avoid calling capable() unconditionally > > > > > > > in simple_xattr_list(), which causes issues under SELinux (see > > > > > > > explanation in the second patch). > > > > > > > > > > > > > > The first patch tries to make this change safer by converting > > > > > > > simple_xattrs to use the RCU mechanism, so that capable() is not called > > > > > > > while the xattrs->lock is held. I didn't find evidence that this is an > > > > > > > issue in the current code, but it can't hurt to make that change > > > > > > > either way (and it was quite straightforward). > > > > > > > > > > > > Hey Ondrey, > > > > > > > > > > > > There's another patchset I'd like to see first which switches from a > > > > > > linked list to an rbtree to get rid of performance issues in this code > > > > > > that can be used to dos tmpfs in containers: > > > > > > > > > > > > https://lore.kernel.org/lkml/d73bd478-e373-f759-2acb-2777f6bba06f@openvz.org > > > > > > > > > > > > I don't think Vasily has time to continue with this so I'll just pick it > > > > > > up hopefully this or the week after LPC. > > > > > > > > > > Hm... does rbtree support lockless traversal? Because if not, that > > > > > > > > The rfc that Vasily sent didn't allow for that at least. > > > > > > > > > would make it impossible to fix the issue without calling capable() > > > > > inside the critical section (or doing something complicated), AFAICT. > > > > > Would rhashtable be a workable alternative to rbtree for this use > > > > > case? Skimming <linux/rhashtable.h> it seems to support both lockless > > > > > lookup and traversal using RCU. And according to its manpage, > > > > > *listxattr(2) doesn't guarantee that the returned names are sorted. > > > > > > > > I've never used the rhashtable infrastructure in any meaningful way. All > > > > I can say from looking at current users that it looks like it could work > > > > well for us here: > > > > > > > > struct simple_xattr { > > > > struct rhlist_head rhlist_head; > > > > char *name; > > > > size_t size; > > > > char value[]; > > > > }; > > > > > > > > static const struct rhashtable_params simple_xattr_rhashtable = { > > > > .head_offset = offsetof(struct simple_xattr, rhlist_head), > > > > .key_offset = offsetof(struct simple_xattr, name), > > > > > > > > or sm like this. > > > > > > I have a patch in rough shape that converts struct simple_xattr to use > > > an rhashtable: > > > > > > https://gitlab.com/brauner/linux/-/commits/fs.xattr.simple.rework/ > > > > > > Light testing, not a lot useful comments and no meaningful commit > > > message as of yet but I'll get to that. > > > > Looks mostly good at first glance. I left comments for some minor > > stuff I noticed. > > > > > Even though your issue is orthogonal to the performance issues I'm > > > trying to fix I went back to your patch, Ondrej to apply it on top. > > > But I think it has one problem. > > > > > > Afaict, by moving the capable() call from the top of the function into > > > the actual traversal portion an unprivileged user can potentially learn > > > whether a file has trusted.* xattrs set. At least if dmesg isn't > > > restricted on the kernel. That may very well be the reason why the > > > capable() call is on top. > > > > Technically it would be possible, for example with SELinux if the > > audit daemon is dead. Not a likely situation, but I agree it's better > > to be safe. > > > > > (Because the straightforward fix for this would be to just call > > > capable() a single time if at least one trusted xattr is encountered and > > > store the result. That's pretty easy to do by making turning the trusted > > > variable into an int, setting it to -1, and only if it's -1 and a > > > trusted xattr has been found call capable() and store the result.) > > > > That would also run into the conundrum of holding a lock while > > (potentially) calling into the LSM subsystem. And would it even fix > > the information leak? Unless I'm missing something it would only > > prevent a leak of the trusted xattr count, but not the presence of any > > trusted xattr. > > No it wouldn't. I just meant this to illustrate that with your patch we > could've made it so that capable() would've only been called once. > > > > > > One option to fix all of that is to switch simple_xattr_list() to use > > > > > > ns_capable_noaudit(&init_user_ns, CAP_SYS_ADMIN) > > > > > > which doesn't generate an audit event. > > > > > > I think this is even the correct thing to do as listing xattrs isn't a > > > targeted operation. IOW, if the the user had used getxattr() to request > > > a trusted.* xattr then logging a denial makes sense as the user > > > explicitly wanted to retrieve a trusted.* xattr. But if the user just > > > requested to list all xattrs then silently skipping trusted without > > > logging an explicit denial xattrs makes sense. > > > > > > Does that sound acceptable? > > > > Yes, I can't see any reason why that wouldn't be the best solution. > > Why haven't I thought of that? :) > > > > I guess you will want to submit a patch for it along with your > > rhashtable patch to avoid a conflict? Or would you like me to submit > > it separately? > > I think you can send a patch for this separately as we don't need to > massage the data structure for this. Ok, will do. > I think we can reasonably give this a > > Fixes: 38f38657444d ("xattr: extract simple_xattr code from tmpfs") # no backport > > But note the "# no backport" as imho it isn't worth backporting this to > older kernels unless that's really desirable. Actually, it would be valuable to have it backported to linux-stable at least, since we have users encountering this on Fedora: https://bugzilla.redhat.com/show_bug.cgi?id=2122888 In the end it's up to the backporter to assess each commit, but at least I wouldn't want to outright discourage the backport in the commit message.