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

Al Viro
2022-09-27T22:51:24Z

[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.

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