The only-root-readable /sys/module/$module/sections/$section files did not truncate their output to the available buffer size. While most paths into the kernfs read handlers end up using PAGE_SIZE buffers, it's possible to get there through other paths (e.g. splice, sendfile). Actually limit the output to the "count" passed into the read function, and report it back correctly. *sigh* Reported-by: kernel test robot <lkp@xxxxxxxxx> Link: https://lore.kernel.org/lkml/20200805002015.GE23458@shao2-debian Fixes: ed66f991bb19 ("module: Refactor section attr into bin attribute") Cc: stable@xxxxxxxxxxxxxxx Cc: Jessica Yu <jeyu@xxxxxxxxxx> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> --- kernel/module.c | 22 +++++++++++++++++++--- 1 file changed, 19 insertions(+), 3 deletions(-) diff --git a/kernel/module.c b/kernel/module.c index aa183c9ac0a2..08c46084d8cc 100644 --- a/kernel/module.c +++ b/kernel/module.c @@ -1520,18 +1520,34 @@ struct module_sect_attrs { struct module_sect_attr attrs[]; }; +#define MODULE_SECT_READ_SIZE (3 /* "0x", "\n" */ + (BITS_PER_LONG / 4)) static ssize_t module_sect_read(struct file *file, struct kobject *kobj, struct bin_attribute *battr, char *buf, loff_t pos, size_t count) { struct module_sect_attr *sattr = container_of(battr, struct module_sect_attr, battr); + char bounce[MODULE_SECT_READ_SIZE + 1]; + size_t wrote; if (pos != 0) return -EINVAL; - return sprintf(buf, "0x%px\n", - kallsyms_show_value(file->f_cred) ? (void *)sattr->address : NULL); + /* + * Since we're a binary read handler, we must account for the + * trailing NUL byte that sprintf will write: if "buf" is + * too small to hold the NUL, or the NUL is exactly the last + * byte, the read will look like it got truncated by one byte. + * Since there is no way to ask sprintf nicely to not write + * the NUL, we have to use a bounce buffer. + */ + wrote = scnprintf(bounce, sizeof(bounce), "0x%px\n", + kallsyms_show_value(file->f_cred) + ? (void *)sattr->address : NULL); + count = min(count, wrote); + memcpy(buf, bounce, count); + + return count; } static void free_sect_attrs(struct module_sect_attrs *sect_attrs) @@ -1580,7 +1596,7 @@ static void add_sect_attrs(struct module *mod, const struct load_info *info) goto out; sect_attrs->nsections++; sattr->battr.read = module_sect_read; - sattr->battr.size = 3 /* "0x", "\n" */ + (BITS_PER_LONG / 4); + sattr->battr.size = MODULE_SECT_READ_SIZE; sattr->battr.attr.mode = 0400; *(gattr++) = &(sattr++)->battr; } -- 2.25.1
