Date: Tue, 4 Dec 2018 16:03:04 +0000 From: Will Deacon <will.deacon@....com> To: Nadav Amit <nadav.amit@...il.com> Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>, Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>, linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, naveen.n.rao@...ux.vnet.ibm.com, anil.s.keshavamurthy@...el.com, David Miller <davem@...emloft.net>, Masami Hiramatsu <mhiramat@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...hat.com>, ast@...nel.org, Daniel Borkmann <daniel@...earbox.net>, jeyu@...nel.org, netdev@...r.kernel.org, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Jann Horn <jannh@...gle.com>, kristen@...ux.intel.com, Dave Hansen <dave.hansen@...el.com>, deneen.t.dock@...el.com, Peter Zijlstra <peterz@...radead.org> Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages On Mon, Dec 03, 2018 at 05:43:11PM -0800, Nadav Amit wrote: > > On Nov 27, 2018, at 4:07 PM, Rick Edgecombe <rick.p.edgecombe@...el.com> wrote: > > > > Since vfree will lazily flush the TLB, but not lazily free the underlying pages, > > it often leaves stale TLB entries to freed pages that could get re-used. This is > > undesirable for cases where the memory being freed has special permissions such > > as executable. > > So I am trying to finish my patch-set for preventing transient W+X mappings > from taking space, by handling kprobes & ftrace that I missed (thanks again for > pointing it out). > > But all of the sudden, I don’t understand why we have the problem that this > (your) patch-set deals with at all. We already change the mappings to make > the memory writable before freeing the memory, so why can’t we make it > non-executable at the same time? Actually, why do we make the module memory, > including its data executable before freeing it??? Yeah, this is really confusing, but I have a suspicion it's a combination of the various different configurations and hysterical raisins. We can't rely on module_alloc() allocating from the vmalloc area (see nios2) nor can we rely on disable_ro_nx() being available at build time. If we *could* rely on module allocations always using vmalloc(), then we could pass in Rick's new flag and drop disable_ro_nx() altogether afaict -- who cares about the memory attributes of a mapping that's about to disappear anyway? Is it just nios2 that does something different? Will
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