Date: Tue, 27 Mar 2018 16:55:12 -0700 From: Kees Cook <keescook@...omium.org> To: Andrew Morton <akpm@...ux-foundation.org> Cc: LKML <linux-kernel@...r.kernel.org>, Segher Boessenkool <segher@...nel.crashing.org>, Thomas Gleixner <tglx@...utronix.de>, Kernel Hardening <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH v2] rslib: Remove VLAs by setting upper bound on nroots On Tue, Mar 27, 2018 at 4:45 PM, Andrew Morton <akpm@...ux-foundation.org> wrote: > On Mon, 26 Mar 2018 16:17:57 -0700 Kees Cook <keescook@...omium.org> wrote: > >> On Fri, Mar 16, 2018 at 11:25 PM, Kees Cook <keescook@...omium.org> wrote: >> > On Fri, Mar 16, 2018 at 3:59 PM, Andrew Morton >> > <akpm@...ux-foundation.org> wrote: >> >> On Thu, 15 Mar 2018 15:59:19 -0700 Kees Cook <keescook@...omium.org> wrote: >> >> >> >>> Avoid stack VLAs by always allocating the upper bound of stack space >> >>> needed. The existing users of rslib appear to max out at 24 roots, >> >>> so use that as the upper bound until we have a reason to change it. >> >>> >> >>> Alternative considered: make init_rs() a true caller-instance and >> >>> pre-allocate the workspaces. This would possibly need locking and >> >>> a refactoring of the returned structure. >> >>> >> >>> Using kmalloc in this path doesn't look great, especially since at >> >>> least one caller (pstore) is sensitive to allocations during rslib >> >>> usage (it expects to run it during an Oops, for example). >> >> >> >> Oh. >> >> >> >> Could we allocate the storage during init_rs(), attach it to `struct >> >> rs_control'? >> > >> > No, because they're modified during decode, and struct rs_control is >> > shared between users. :( >> > >> > Doing those changes is possible, but it requires a rather extensive >> > analysis of callers, etc. >> > >> > Hence, the 24 ultimately. >> >> Can this land in -mm, or does this need further discussion? > > Grumble. That share-the-rs_control-if-there's-already-a-matching-one > thing looks like premature optimization to me :( > > I guess if we put this storage into the rs_control (rather than on the > stack) then we'd have to worry about concurrent uses of it. It looks > like all the other fields are immutable once it's set up so there might > be such users. In fact, I suspect there are... Exactly. :( This is the same conclusion tglx and I came to. -Kees -- Kees Cook Pixel Security
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