[PATCH 0/2] module: Correctly truncate sysfs sections output

[PATCH 0/2] module: Correctly truncate sysfs sections output

[Kees Cook]

Hi,

This fixes my sysfs module sections refactoring to take into account
the case where the output buffer is not PAGE_SIZE. :( Thanks to 0day
and trinity for noticing.

I'll let this sit in -next for a few days and then send it to Linus.

-Kees

Kees Cook (2):
  module: Correctly truncate sysfs sections output
  selftests: splice: Check behavior of full and short splices

 kernel/module.c                               | 22 ++++++-
 tools/testing/selftests/splice/.gitignore     |  1 +
 tools/testing/selftests/splice/Makefile       |  4 +-
 tools/testing/selftests/splice/config         |  1 +
 tools/testing/selftests/splice/settings       |  1 +
 .../selftests/splice/short_splice_read.sh     | 56 ++++++++++++++++++
 tools/testing/selftests/splice/splice_read.c  | 57 +++++++++++++++++++
 7 files changed, 137 insertions(+), 5 deletions(-)
 create mode 100644 tools/testing/selftests/splice/config
 create mode 100644 tools/testing/selftests/splice/settings
 create mode 100755 tools/testing/selftests/splice/short_splice_read.sh
 create mode 100644 tools/testing/selftests/splice/splice_read.c

-- 
2.25.1

[PATCH 1/2] module: Correctly truncate sysfs sections output

[Kees Cook]

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@intel.com>
Link: https://lore.kernel.org/lkml/20200805002015.GE23458@shao2-debian
Fixes: ed66f991bb19 ("module: Refactor section attr into bin attribute")
Cc: stable@vger.kernel.org
Cc: Jessica Yu <jeyu@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 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

Re: [PATCH 1/2] module: Correctly truncate sysfs sections output

[Greg Kroah-Hartman]

On Thu, Aug 06, 2020 at 11:35:38PM -0700, Kees Cook wrote:
> 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*

Ugh, never thought about that...

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Re: [PATCH 1/2] module: Correctly truncate sysfs sections output

[Jessica Yu]

+++ Kees Cook [06/08/20 23:35 -0700]:
>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@intel.com>
>Link: https://lore.kernel.org/lkml/20200805002015.GE23458@shao2-debian
>Fixes: ed66f991bb19 ("module: Refactor section attr into bin attribute")
>Cc: stable@vger.kernel.org
>Cc: Jessica Yu <jeyu@kernel.org>
>Signed-off-by: Kees Cook <keescook@chromium.org>

Oof, thanks for fixing this!

Acked-by: Jessica Yu <jeyu@kernel.org>

>---
> 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
>

[PATCH 2/2] selftests: splice: Check behavior of full and short splices

[Kees Cook]

In order to help catch regressions in splice vs read behavior in certain
special files, test a few with various different kinds of internal
kernel helpers.

Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 tools/testing/selftests/splice/.gitignore     |  1 +
 tools/testing/selftests/splice/Makefile       |  4 +-
 tools/testing/selftests/splice/config         |  1 +
 tools/testing/selftests/splice/settings       |  1 +
 .../selftests/splice/short_splice_read.sh     | 56 ++++++++++++++++++
 tools/testing/selftests/splice/splice_read.c  | 57 +++++++++++++++++++
 6 files changed, 118 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/splice/config
 create mode 100644 tools/testing/selftests/splice/settings
 create mode 100755 tools/testing/selftests/splice/short_splice_read.sh
 create mode 100644 tools/testing/selftests/splice/splice_read.c

diff --git a/tools/testing/selftests/splice/.gitignore b/tools/testing/selftests/splice/.gitignore
index d5a2da428752..be8266f5d04c 100644
--- a/tools/testing/selftests/splice/.gitignore
+++ b/tools/testing/selftests/splice/.gitignore
@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0-only
 default_file_splice_read
+splice_read
diff --git a/tools/testing/selftests/splice/Makefile b/tools/testing/selftests/splice/Makefile
index e519b159b60d..541cd826d5a5 100644
--- a/tools/testing/selftests/splice/Makefile
+++ b/tools/testing/selftests/splice/Makefile
@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0
-TEST_PROGS := default_file_splice_read.sh
-TEST_GEN_PROGS_EXTENDED := default_file_splice_read
+TEST_PROGS := default_file_splice_read.sh short_splice_read.sh
+TEST_GEN_PROGS_EXTENDED := default_file_splice_read splice_read
 
 include ../lib.mk
diff --git a/tools/testing/selftests/splice/config b/tools/testing/selftests/splice/config
new file mode 100644
index 000000000000..058c928368b8
--- /dev/null
+++ b/tools/testing/selftests/splice/config
@@ -0,0 +1 @@
+CONFIG_TEST_LKM=m
diff --git a/tools/testing/selftests/splice/settings b/tools/testing/selftests/splice/settings
new file mode 100644
index 000000000000..89cedfc0d12b
--- /dev/null
+++ b/tools/testing/selftests/splice/settings
@@ -0,0 +1 @@
+timeout=5
diff --git a/tools/testing/selftests/splice/short_splice_read.sh b/tools/testing/selftests/splice/short_splice_read.sh
new file mode 100755
index 000000000000..7810d3589d9a
--- /dev/null
+++ b/tools/testing/selftests/splice/short_splice_read.sh
@@ -0,0 +1,56 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+set -e
+
+ret=0
+
+do_splice()
+{
+	filename="$1"
+	bytes="$2"
+	expected="$3"
+
+	out=$(./splice_read "$filename" "$bytes" | cat)
+	if [ "$out" = "$expected" ] ; then
+		echo "ok: $filename $bytes"
+	else
+		echo "FAIL: $filename $bytes"
+		ret=1
+	fi
+}
+
+test_splice()
+{
+	filename="$1"
+
+	full=$(cat "$filename")
+	two=$(echo "$full" | grep -m1 . | cut -c-2)
+
+	# Make sure full splice has the same contents as a standard read.
+	do_splice "$filename" 4096 "$full"
+
+	# Make sure a partial splice see the first two characters.
+	do_splice "$filename" 2 "$two"
+}
+
+# proc_single_open(), seq_read()
+test_splice /proc/$$/limits
+# special open, seq_read()
+test_splice /proc/$$/comm
+
+# proc_handler, proc_dointvec_minmax
+test_splice /proc/sys/fs/nr_open
+# proc_handler, proc_dostring
+test_splice /proc/sys/kernel/modprobe
+# proc_handler, special read
+test_splice /proc/sys/kernel/version
+
+if ! [ -d /sys/module/test_module/sections ] ; then
+	modprobe test_module
+fi
+# kernfs, attr
+test_splice /sys/module/test_module/coresize
+# kernfs, binattr
+test_splice /sys/module/test_module/sections/.init.text
+
+exit $ret
diff --git a/tools/testing/selftests/splice/splice_read.c b/tools/testing/selftests/splice/splice_read.c
new file mode 100644
index 000000000000..46dae6a25cfb
--- /dev/null
+++ b/tools/testing/selftests/splice/splice_read.c
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+
+int main(int argc, char *argv[])
+{
+	int fd;
+	size_t size;
+	ssize_t spliced;
+
+	if (argc < 2) {
+		fprintf(stderr, "Usage: %s INPUT [BYTES]\n", argv[0]);
+		return EXIT_FAILURE;
+	}
+
+	fd = open(argv[1], O_RDONLY);
+	if (fd < 0) {
+		perror(argv[1]);
+		return EXIT_FAILURE;
+	}
+
+	if (argc == 3)
+		size = atol(argv[2]);
+	else {
+		struct stat statbuf;
+
+		if (fstat(fd, &statbuf) < 0) {
+			perror(argv[1]);
+			return EXIT_FAILURE;
+		}
+
+		if (statbuf.st_size > INT_MAX) {
+			fprintf(stderr, "%s: Too big\n", argv[1]);
+			return EXIT_FAILURE;
+		}
+
+		size = statbuf.st_size;
+	}
+
+	/* splice(2) file to stdout. */
+	spliced = splice(fd, NULL, STDOUT_FILENO, NULL,
+		      size, SPLICE_F_MOVE);
+	if (spliced < 0) {
+		perror("splice");
+		return EXIT_FAILURE;
+	}
+
+	close(fd);
+	return EXIT_SUCCESS;
+}
-- 
2.25.1