Re: [PATCH][CFT] [coredump] don't use __kernel_write() on kmap_local_page()

Eric W. Biederman
2022-09-28T17:58:23Z

Al Viro <viro@zeniv.linux.org.uk> writes:

> [I'm going to send a pull request tomorrow if nobody yells;
> please review and test - it seems to work fine here, but extra
> eyes and extra testing would be very welcome]
>
> passing kmap_local_page() result to __kernel_write() is unsafe -
> random ->write_iter() might (and 9p one does) get unhappy when
> passed ITER_KVEC with pointer that came from kmap_local_page().
>
> Fix by providing a variant of __kernel_write() that takes an iov_iter
> from caller (__kernel_write() becomes a trivial wrapper) and adding
> dump_emit_page() that parallels dump_emit(), except that instead of
> __kernel_write() it uses __kernel_write_iter() with ITER_BVEC source.

Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>

From a 10,000 foot view this makes sense.

> Fixes: 3159ed57792b "fs/coredump: use kmap_local_page()"
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
> diff --git a/fs/coredump.c b/fs/coredump.c
> index 9f4aae202109..09dbc981ad5c 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -832,6 +832,38 @@ static int __dump_skip(struct coredump_params *cprm, size_t nr)
> }
> }
>
> +static int dump_emit_page(struct coredump_params *cprm, struct page *page)
> +{
> + struct bio_vec bvec = {
> + .bv_page = page,
> + .bv_offset = 0,
> + .bv_len = PAGE_SIZE,
> + };
> + struct iov_iter iter;
> + struct file *file = cprm->file;
> + loff_t pos = file->f_pos;
> + ssize_t n;
> +
> + if (cprm->to_skip) {
> + if (!__dump_skip(cprm, cprm->to_skip))
> + return 0;
> + cprm->to_skip = 0;
> + }
> + if (cprm->written + PAGE_SIZE > cprm->limit)
> + return 0;
> + if (dump_interrupted())
> + return 0;
> + iov_iter_bvec(&iter, WRITE, &bvec, 1, PAGE_SIZE);
> + n = __kernel_write_iter(cprm->file, &iter, &pos);
> + if (n != PAGE_SIZE)
> + return 0;
> + file->f_pos = pos;
> + cprm->written += PAGE_SIZE;
> + cprm->pos += PAGE_SIZE;
> +
> + return 1;
> +}
> +
> int dump_emit(struct coredump_params *cprm, const void *addr, int nr)
> {
> if (cprm->to_skip) {
> @@ -863,7 +895,6 @@ int dump_user_range(struct coredump_params *cprm, unsigned long start,
>
> for (addr = start; addr < start + len; addr += PAGE_SIZE) {
> struct page *page;
> - int stop;
>
> /*
> * To avoid having to allocate page tables for virtual address
> @@ -874,12 +905,7 @@ int dump_user_range(struct coredump_params *cprm, unsigned long start,
> */
> page = get_dump_page(addr);
> if (page) {
> - void *kaddr = kmap_local_page(page);
> -
> - stop = !dump_emit(cprm, kaddr, PAGE_SIZE);
> - kunmap_local(kaddr);
> - put_page(page);
> - if (stop)
> + if (!dump_emit_page(cprm, page))
> return 0;
> } else {
> dump_skip(cprm, PAGE_SIZE);
> diff --git a/fs/internal.h b/fs/internal.h
> index 87e96b9024ce..3e206d3e317c 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -16,6 +16,7 @@ struct shrink_control;
> struct fs_context;
> struct user_namespace;
> struct pipe_inode_info;
> +struct iov_iter;
>
> /*
> * block/bdev.c
> @@ -221,3 +222,5 @@ ssize_t do_getxattr(struct user_namespace *mnt_userns,
> int setxattr_copy(const char __user *name, struct xattr_ctx *ctx);
> int do_setxattr(struct user_namespace *mnt_userns, struct dentry *dentry,
> struct xattr_ctx *ctx);
> +
> +ssize_t __kernel_write_iter(struct file *file, struct iov_iter *from, loff_t *pos);
> diff --git a/fs/read_write.c b/fs/read_write.c
> index 1a261dcf1778..328ce8cf9a85 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -496,14 +496,9 @@ static ssize_t new_sync_write(struct file *filp, const char __user *buf, size_t
> }
>
> /* caller is responsible for file_start_write/file_end_write */
> -ssize_t __kernel_write(struct file *file, const void *buf, size_t count, loff_t *pos)
> +ssize_t __kernel_write_iter(struct file *file, struct iov_iter *from, loff_t *pos)
> {
> - struct kvec iov = {
> - .iov_base = (void *)buf,
> - .iov_len = min_t(size_t, count, MAX_RW_COUNT),
> - };
> struct kiocb kiocb;
> - struct iov_iter iter;
> ssize_t ret;
>
> if (WARN_ON_ONCE(!(file->f_mode & FMODE_WRITE)))
> @@ -519,8 +514,7 @@ ssize_t __kernel_write(struct file *file, const void *buf, size_t count, loff_t
>
> init_sync_kiocb(&kiocb, file);
> kiocb.ki_pos = pos ? *pos : 0;
> - iov_iter_kvec(&iter, WRITE, &iov, 1, iov.iov_len);
> - ret = file->f_op->write_iter(&kiocb, &iter);
> + ret = file->f_op->write_iter(&kiocb, from);
> if (ret > 0) {
> if (pos)
> *pos = kiocb.ki_pos;
> @@ -530,6 +524,18 @@ ssize_t __kernel_write(struct file *file, const void *buf, size_t count, loff_t
> inc_syscw(current);
> return ret;
> }
> +
> +/* caller is responsible for file_start_write/file_end_write */
> +ssize_t __kernel_write(struct file *file, const void *buf, size_t count, loff_t *pos)
> +{
> + struct kvec iov = {
> + .iov_base = (void *)buf,
> + .iov_len = min_t(size_t, count, MAX_RW_COUNT),
> + };
> + struct iov_iter iter;
> + iov_iter_kvec(&iter, WRITE, &iov, 1, iov.iov_len);
> + return __kernel_write_iter(file, &iter, pos);
> +}
> /*
> * This "EXPORT_SYMBOL_GPL()" is more of a "EXPORT_SYMBOL_DONTUSE()",
> * but autofs is one of the few internal kernel users that actually