[PATCH v3 16/19] fs/kernel_file_read: Add "offset" arg for partial reads

Kees Cook
2020-07-24T21:43:43Z

To perform partial reads, callers of kernel_read_file*() must have a
non-NULL file_size argument and a preallocated buffer. The new "offset"
argument can then be used to seek to specific locations in the file to
fill the buffer to, at most, "buf_size" per call.

Where possible, the LSM hooks can report whether a full file has been
read or not so that the contents can be reasoned about.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
drivers/base/firmware_loader/main.c | 2 +-
fs/kernel_read_file.c | 78 ++++++++++++++++++++---------
include/linux/kernel_read_file.h | 8 +--
kernel/kexec_file.c | 4 +-
kernel/module.c | 2 +-
security/integrity/digsig.c | 2 +-
security/integrity/ima/ima_fs.c | 3 +-
7 files changed, 65 insertions(+), 34 deletions(-)

diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
index bd199404935f..d95249b5284e 100644
--- a/drivers/base/firmware_loader/main.c
+++ b/drivers/base/firmware_loader/main.c
@@ -494,7 +494,7 @@ fw_get_filesystem_firmware(struct device *device, struct fw_priv *fw_priv,
fw_priv->size = 0;

/* load firmware files from the mount namespace of init */
- rc = kernel_read_file_from_path_initns(path, &buffer, msize,
+ rc = kernel_read_file_from_path_initns(path, 0, &buffer, msize,
NULL,
READING_FIRMWARE);
if (rc < 0) {
diff --git a/fs/kernel_read_file.c b/fs/kernel_read_file.c
index d73bc3fa710a..90d255fbdd9b 100644
--- a/fs/kernel_read_file.c
+++ b/fs/kernel_read_file.c
@@ -9,6 +9,7 @@
* kernel_read_file() - read file contents into a kernel buffer
*
* @file file to read from
+ * @offset where to start reading from (see below).
* @buf pointer to a "void *" buffer for reading into (if
* *@buf is NULL, a buffer will be allocated, and
* @buf_size will be ignored)
@@ -19,19 +20,31 @@
* @id the kernel_read_file_id identifying the type of
* file contents being read (for LSMs to examine)
*
+ * @offset must be 0 unless both @buf and @file_size are non-NULL
+ * (i.e. the caller must be expecting to read partial file contents
+ * via an already-allocated @buf, in at most @buf_size chunks, and
+ * will be able to determine when the entire file was read by
+ * checking @file_size). This isn't a recommended way to read a
+ * file, though, since it is possible that the contents might
+ * change between calls to kernel_read_file().
+ *
* Returns number of bytes read (no single read will be bigger
* than INT_MAX), or negative on error.
*
*/
-int kernel_read_file(struct file *file, void **buf,
+int kernel_read_file(struct file *file, loff_t offset, void **buf,
size_t buf_size, size_t *file_size,
enum kernel_read_file_id id)
{
loff_t i_size, pos;
- ssize_t bytes = 0;
+ size_t copied;
void *allocated = NULL;
+ bool whole_file;
int ret;

+ if (offset != 0 && (!*buf || !file_size))
+ return -EINVAL;
+
if (!S_ISREG(file_inode(file)->i_mode))
return -EINVAL;

@@ -39,19 +52,27 @@ int kernel_read_file(struct file *file, void **buf,
if (ret)
return ret;

- ret = security_kernel_read_file(file, id, true);
- if (ret)
- goto out;
-
i_size = i_size_read(file_inode(file));
if (i_size <= 0) {
ret = -EINVAL;
goto out;
}
- if (i_size > INT_MAX || i_size > buf_size) {
+ /* The file is too big for sane activities. */
+ if (i_size > INT_MAX) {
+ ret = -EFBIG;
+ goto out;
+ }
+ /* The entire file cannot be read in one buffer. */
+ if (!file_size && offset == 0 && i_size > buf_size) {
ret = -EFBIG;
goto out;
}
+
+ whole_file = (offset == 0 && i_size <= buf_size);
+ ret = security_kernel_read_file(file, id, whole_file);
+ if (ret)
+ goto out;
+
if (file_size)
*file_size = i_size;

@@ -62,9 +83,14 @@ int kernel_read_file(struct file *file, void **buf,
goto out;
}

- pos = 0;
- while (pos < i_size) {
- bytes = kernel_read(file, *buf + pos, i_size - pos, &pos);
+ pos = offset;
+ copied = 0;
+ while (copied < buf_size) {
+ ssize_t bytes;
+ size_t wanted = min_t(size_t, buf_size - copied,
+ i_size - pos);
+
+ bytes = kernel_read(file, *buf + copied, wanted, &pos);
if (bytes < 0) {
ret = bytes;
goto out_free;
@@ -72,14 +98,17 @@ int kernel_read_file(struct file *file, void **buf,

if (bytes == 0)
break;
+ copied += bytes;
}

- if (pos != i_size) {
- ret = -EIO;
- goto out_free;
- }
+ if (whole_file) {
+ if (pos != i_size) {
+ ret = -EIO;
+ goto out_free;
+ }

- ret = security_kernel_post_read_file(file, *buf, i_size, id);
+ ret = security_kernel_post_read_file(file, *buf, i_size, id);
+ }

out_free:
if (ret < 0) {
@@ -91,11 +120,11 @@ int kernel_read_file(struct file *file, void **buf,

out:
allow_write_access(file);
- return ret == 0 ? pos : ret;
+ return ret == 0 ? copied : ret;
}
EXPORT_SYMBOL_GPL(kernel_read_file);

-int kernel_read_file_from_path(const char *path, void **buf,
+int kernel_read_file_from_path(const char *path, loff_t offset, void **buf,
size_t buf_size, size_t *file_size,
enum kernel_read_file_id id)
{
@@ -109,14 +138,15 @@ int kernel_read_file_from_path(const char *path, void **buf,
if (IS_ERR(file))
return PTR_ERR(file);

- ret = kernel_read_file(file, buf, buf_size, file_size, id);
+ ret = kernel_read_file(file, offset, buf, buf_size, file_size, id);
fput(file);
return ret;
}
EXPORT_SYMBOL_GPL(kernel_read_file_from_path);

-int kernel_read_file_from_path_initns(const char *path, void **buf,
- size_t buf_size, size_t *file_size,
+int kernel_read_file_from_path_initns(const char *path, loff_t offset,
+ void **buf, size_t buf_size,
+ size_t *file_size,
enum kernel_read_file_id id)
{
struct file *file;
@@ -135,14 +165,14 @@ int kernel_read_file_from_path_initns(const char *path, void **buf,
if (IS_ERR(file))
return PTR_ERR(file);

- ret = kernel_read_file(file, buf, buf_size, file_size, id);
+ ret = kernel_read_file(file, offset, buf, buf_size, file_size, id);
fput(file);
return ret;
}
EXPORT_SYMBOL_GPL(kernel_read_file_from_path_initns);

-int kernel_read_file_from_fd(int fd, void **buf, size_t buf_size,
- size_t *file_size,
+int kernel_read_file_from_fd(int fd, loff_t offset, void **buf,
+ size_t buf_size, size_t *file_size,
enum kernel_read_file_id id)
{
struct fd f = fdget(fd);
@@ -151,7 +181,7 @@ int kernel_read_file_from_fd(int fd, void **buf, size_t buf_size,
if (!f.file)
goto out;

- ret = kernel_read_file(f.file, buf, buf_size, file_size, id);
+ ret = kernel_read_file(f.file, offset, buf, buf_size, file_size, id);
out:
fdput(f);
return ret;
diff --git a/include/linux/kernel_read_file.h b/include/linux/kernel_read_file.h
index 023293eaf948..575ffa1031d3 100644
--- a/include/linux/kernel_read_file.h
+++ b/include/linux/kernel_read_file.h
@@ -35,19 +35,19 @@ static inline const char *kernel_read_file_id_str(enum kernel_read_file_id id)
return kernel_read_file_str[id];
}

-int kernel_read_file(struct file *file,
+int kernel_read_file(struct file *file, loff_t offset,
void **buf, size_t buf_size,
size_t *file_size,
enum kernel_read_file_id id);
-int kernel_read_file_from_path(const char *path,
+int kernel_read_file_from_path(const char *path, loff_t offset,
void **buf, size_t buf_size,
size_t *file_size,
enum kernel_read_file_id id);
-int kernel_read_file_from_path_initns(const char *path,
+int kernel_read_file_from_path_initns(const char *path, loff_t offset,
void **buf, size_t buf_size,
size_t *file_size,
enum kernel_read_file_id id);
-int kernel_read_file_from_fd(int fd,
+int kernel_read_file_from_fd(int fd, loff_t offset,
void **buf, size_t buf_size,
size_t *file_size,
enum kernel_read_file_id id);
diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 878ca684a3a1..45726bc8f6ce 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -221,7 +221,7 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd,
int ret;
void *ldata;

- ret = kernel_read_file_from_fd(kernel_fd, &image->kernel_buf,
+ ret = kernel_read_file_from_fd(kernel_fd, 0, &image->kernel_buf,
INT_MAX, NULL, READING_KEXEC_IMAGE);
if (ret < 0)
return ret;
@@ -241,7 +241,7 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd,
#endif
/* It is possible that there no initramfs is being loaded */
if (!(flags & KEXEC_FILE_NO_INITRAMFS)) {
- ret = kernel_read_file_from_fd(initrd_fd, &image->initrd_buf,
+ ret = kernel_read_file_from_fd(initrd_fd, 0, &image->initrd_buf,
INT_MAX, NULL,
READING_KEXEC_INITRAMFS);
if (ret < 0)
diff --git a/kernel/module.c b/kernel/module.c
index 90a4788dff9d..d353d1f67681 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -4007,7 +4007,7 @@ SYSCALL_DEFINE3(finit_module, int, fd, const char __user *, uargs, int, flags)
|MODULE_INIT_IGNORE_VERMAGIC))
return -EINVAL;

- err = kernel_read_file_from_fd(fd, &hdr, INT_MAX, NULL,
+ err = kernel_read_file_from_fd(fd, 0, &hdr, INT_MAX, NULL,
READING_MODULE);
if (err < 0)
return err;
diff --git a/security/integrity/digsig.c b/security/integrity/digsig.c
index 8a523dfd7fd7..0f518dcfde05 100644
--- a/security/integrity/digsig.c
+++ b/security/integrity/digsig.c
@@ -175,7 +175,7 @@ int __init integrity_load_x509(const unsigned int id, const char *path)
int rc;
key_perm_t perm;

- rc = kernel_read_file_from_path(path, &data, INT_MAX, NULL,
+ rc = kernel_read_file_from_path(path, 0, &data, INT_MAX, NULL,
READING_X509_CERTIFICATE);
if (rc < 0) {
pr_err("Unable to open file: %s (%d)", path, rc);
diff --git a/security/integrity/ima/ima_fs.c b/security/integrity/ima/ima_fs.c
index 5fc56ccb6678..ea8ff8a07b36 100644
--- a/security/integrity/ima/ima_fs.c
+++ b/security/integrity/ima/ima_fs.c
@@ -284,7 +284,8 @@ static ssize_t ima_read_policy(char *path)
datap = path;
strsep(&datap, "\n");

- rc = kernel_read_file_from_path(path, &data, INT_MAX, NULL, READING_POLICY);
+ rc = kernel_read_file_from_path(path, 0, &data, INT_MAX, NULL,
+ READING_POLICY);
if (rc < 0) {
pr_err("Unable to open file: %s (%d)", path, rc);
return rc;
--
2.25.1