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    SubjectRe: [PATCH v4 09/10] kallsyms: Hide layout
    On Fri, Jul 17, 2020 at 10:00:06AM -0700, Kristen Carlson Accardi wrote:
    > This patch makes /proc/kallsyms display in a random order, rather
    > than sorted by address in order to hide the newly randomized address
    > layout.

    Ah! Much nicer. Is there any reason not to just do this unconditionally,
    regardless of FGKASLR? It's a smallish dynamic allocation, and
    displaying kallsyms is hardly fast-path...

    >
    > Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
    > Reviewed-by: Tony Luck <tony.luck@intel.com>
    > Tested-by: Tony Luck <tony.luck@intel.com>
    > ---
    > kernel/kallsyms.c | 163 +++++++++++++++++++++++++++++++++++++++++++++-
    > 1 file changed, 162 insertions(+), 1 deletion(-)
    >
    > diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
    > index bb14e64f62a4..45d147f7f10e 100644
    > --- a/kernel/kallsyms.c
    > +++ b/kernel/kallsyms.c
    > @@ -446,6 +446,12 @@ struct kallsym_iter {
    > int show_value;
    > };
    >
    > +struct kallsyms_shuffled_iter {
    > + struct kallsym_iter iter;
    > + loff_t total_syms;
    > + loff_t shuffled_index[];
    > +};
    > +
    > int __weak arch_get_kallsym(unsigned int symnum, unsigned long *value,
    > char *type, char *name)
    > {
    > @@ -661,7 +667,7 @@ bool kallsyms_show_value(const struct cred *cred)
    > }
    > }
    >
    > -static int kallsyms_open(struct inode *inode, struct file *file)
    > +static int __kallsyms_open(struct inode *inode, struct file *file)
    > {
    > /*
    > * We keep iterator in m->private, since normal case is to
    > @@ -682,6 +688,161 @@ static int kallsyms_open(struct inode *inode, struct file *file)
    > return 0;
    > }
    >
    > +/*
    > + * When function granular kaslr is enabled, we need to print out the symbols
    > + * at random so we don't reveal the new layout.
    > + */
    > +#if defined(CONFIG_FG_KASLR)
    > +static int update_random_pos(struct kallsyms_shuffled_iter *s_iter,
    > + loff_t pos, loff_t *new_pos)
    > +{
    > + loff_t new;
    > +
    > + if (pos >= s_iter->total_syms)
    > + return 0;
    > +
    > + new = s_iter->shuffled_index[pos];
    > +
    > + /*
    > + * normally this would be done as part of update_iter, however,
    > + * we want to avoid triggering this in the event that new is
    > + * zero since we don't want to blow away our pos end indicators.
    > + */
    > + if (new == 0) {
    > + s_iter->iter.name[0] = '\0';
    > + s_iter->iter.nameoff = get_symbol_offset(new);
    > + s_iter->iter.pos = new;
    > + }
    > +
    > + *new_pos = new;
    > + return 1;
    > +}
    > +
    > +static void *shuffled_start(struct seq_file *m, loff_t *pos)
    > +{
    > + struct kallsyms_shuffled_iter *s_iter = m->private;
    > + loff_t new_pos;
    > +
    > + if (!update_random_pos(s_iter, *pos, &new_pos))
    > + return NULL;
    > +
    > + return s_start(m, &new_pos);
    > +}
    > +
    > +static void *shuffled_next(struct seq_file *m, void *p, loff_t *pos)
    > +{
    > + struct kallsyms_shuffled_iter *s_iter = m->private;
    > + loff_t new_pos;
    > +
    > + (*pos)++;
    > +
    > + if (!update_random_pos(s_iter, *pos, &new_pos))
    > + return NULL;
    > +
    > + if (!update_iter(m->private, new_pos))
    > + return NULL;
    > +
    > + return p;
    > +}
    > +
    > +/*
    > + * shuffle_index_list()
    > + * Use a Fisher Yates algorithm to shuffle a list of text sections.
    > + */
    > +static void shuffle_index_list(loff_t *indexes, loff_t size)
    > +{
    > + int i;
    > + unsigned int j;
    > + loff_t temp;
    > +
    > + for (i = size - 1; i > 0; i--) {
    > + /* pick a random index from 0 to i */
    > + get_random_bytes(&j, sizeof(j));
    > + j = j % (i + 1);
    > +
    > + temp = indexes[i];
    > + indexes[i] = indexes[j];
    > + indexes[j] = temp;
    > + }
    > +}
    > +
    > +static const struct seq_operations kallsyms_shuffled_op = {
    > + .start = shuffled_start,
    > + .next = shuffled_next,
    > + .stop = s_stop,
    > + .show = s_show
    > +};
    > +
    > +static int kallsyms_random_open(struct inode *inode, struct file *file)
    > +{
    > + loff_t pos;
    > + struct kallsyms_shuffled_iter *shuffled_iter;
    > + struct kallsym_iter iter;
    > + bool show_value;
    > +
    > + /*
    > + * If privileged, go ahead and use the normal algorithm for
    > + * displaying symbols
    > + */
    > + show_value = kallsyms_show_value(file->f_cred);
    > + if (show_value)
    > + return __kallsyms_open(inode, file);
    > +
    > + /*
    > + * we need to figure out how many extra symbols there are
    > + * to print out past kallsyms_num_syms
    > + */
    > + pos = kallsyms_num_syms;
    > + reset_iter(&iter, 0);
    > + do {
    > + if (!update_iter(&iter, pos))
    > + break;
    > + pos++;
    > + } while (1);

    Can this be tracked separately instead of needing to search for it every
    time? (Looks like it's modules and ftrace? Could they each have a
    *_num_sysms?)

    (I need to go read how kallsyms doesn't miscount in general when the
    symbol table changes out from under it...)


    --
    Kees Cook

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