Messages in this thread | | | From | Dan Williams <> | Date | Wed, 3 Jan 2018 21:44:33 -0800 | Subject | Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier |
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On Wed, Jan 3, 2018 at 8:44 PM, Al Viro <viro@zeniv.linux.org.uk> wrote: > On Thu, Jan 04, 2018 at 03:10:51AM +0000, Williams, Dan J wrote: > >> diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h >> index 1c65817673db..dbc12007da51 100644 >> --- a/include/linux/fdtable.h >> +++ b/include/linux/fdtable.h >> @@ -82,8 +82,10 @@ static inline struct file *__fcheck_files(struct files_struct *files, unsigned i >> { >> struct fdtable *fdt = rcu_dereference_raw(files->fdt); >> >> - if (fd < fdt->max_fds) >> + if (fd < fdt->max_fds) { >> + osb(); >> return rcu_dereference_raw(fdt->fd[fd]); >> + } >> return NULL; >> } > > ... and the point of that would be? Possibly revealing the value of files->fdt? > Why would that be a threat, assuming you manage to extract the information in > question in the first place?
No, the concern is that an fd value >= fdt->max_fds may cause the cpu to read arbitrary memory addresses relative to files->fdt and userspace can observe that it got loaded. With the barrier the speculation stops and never allows that speculative read to issue. With the change, the cpu only issues a read for fdt->fd[fd] when fd is valid.
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