[PATCH 15/18] vfs, fdtable: prevent bounds-check bypass via speculative execution

Dan Williams
2018-01-06T01:11:15Z

Expectedly, static analysis reports that 'fd' is a user controlled value
that is used as a data dependency to read from the 'fdt->fd' array. In
order to avoid potential leaks of kernel memory values, block
speculative execution of the instruction stream that could issue reads
based on an invalid 'file *' returned from __fcheck_files.

Based on an original patch by Elena Reshetova.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
include/linux/fdtable.h | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 1c65817673db..4a147c5c2533 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -81,9 +81,10 @@ struct dentry;
static inline struct file *__fcheck_files(struct files_struct *files, unsigned int fd)
{
struct fdtable *fdt = rcu_dereference_raw(files->fdt);
+ struct file __rcu **fdp;

- if (fd < fdt->max_fds)
- return rcu_dereference_raw(fdt->fd[fd]);
+ if ((fdp = nospec_array_ptr(fdt->fd, fd, fdt->max_fds)))
+ return rcu_dereference_raw(*fdp);
return NULL;
}