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SubjectRe: [PATCH V37 27/29] tracefs: Restrict tracefs when the kernel is locked down
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Date
Hi

On 2019-08-01 00:16, Matthew Garrett wrote:
> Tracefs may release more information about the kernel than desirable, so
> restrict it when the kernel is locked down in confidentiality mode by
> preventing open().
>
> Signed-off-by: Matthew Garrett <mjg59@google.com>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

This patch causes the following regression on various Samsung Exynos SoC
based boards (ARM 32bit):

[   15.364422] Unable to handle kernel NULL pointer dereference at
virtual address 00000000
[   15.368775] pgd = a530ddbe
[   15.371447] [00000000] *pgd=bcd7c831
[   15.374993] Internal error: Oops: 80000007 [#1] PREEMPT SMP ARM
[   15.380890] Modules linked in:
[   15.383929] CPU: 0 PID: 1393 Comm: perf Not tainted
5.2.0-00027-g757ff7244358-dirty #6459
[   15.392086] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
[   15.398164] PC is at 0x0
[   15.400687] LR is at do_dentry_open+0x22c/0x3b0
[   15.405193] pc : [<00000000>]    lr : [<c02977c4>]    psr: 60000053
[   15.411442] sp : e7317dd8  ip : 00000000  fp : 00000000
[   15.416650] r10: c0187e6c  r9 : c041f8cc  r8 : e72123c8
[   15.421858] r7 : e7317ec0  r6 : e7d89630  r5 : 00000000  r4 : e72123c0
[   15.428368] r3 : 00000000  r2 : 5ba370f3  r1 : e72123c0  r0 : e7d89630
[   15.434880] Flags: nZCv  IRQs on  FIQs off  Mode SVC_32  ISA ARM 
Segment none
[   15.442083] Control: 10c5387d  Table: 6726404a  DAC: 00000051
[   15.447812] Process perf (pid: 1393, stack limit = 0x17621431)
[   15.453628] Stack: (0xe7317dd8 to 0xe7318000)
...
[   15.604842] [<c02977c4>] (do_dentry_open) from [<c02aafc8>]
(path_openat+0x5a0/0x1004)
[   15.612735] [<c02aafc8>] (path_openat) from [<c02acce8>]
(do_filp_open+0x6c/0xd8)
[   15.620200] [<c02acce8>] (do_filp_open) from [<c0298cc4>]
(do_sys_open+0x130/0x1f4)
[   15.627839] [<c0298cc4>] (do_sys_open) from [<c0101000>]
(ret_fast_syscall+0x0/0x28)
[   15.635560] Exception stack(0xe7317fa8 to 0xe7317ff0)
[   15.640596] 7fa0:                   0022dc0b 001deee0 ffffff9c
beb6d764 00020000 00000000
[   15.648756] 7fc0: 0022dc0b 001deee0 0022dba8 00000142 001ba044
00241d68 001a13d8 beb6e78c
[   15.656913] 7fe0: b6f7e000 beb6c6f8 9a27c600 b6f69504
[   15.661952] Code: bad PC value
[   15.665105] ---[ end trace 7e8b864582108f4a ]---

This is standard ARM 32bit kernel with
arch/arm/configs/exynos_defconfig. It is enough to run "perf list" command.

> ---
> fs/tracefs/inode.c | 40 +++++++++++++++++++++++++++++++++++-
> include/linux/security.h | 1 +
> security/lockdown/lockdown.c | 1 +
> 3 files changed, 41 insertions(+), 1 deletion(-)
>
> diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
> index 1387bcd96a79..12a325fb4cbd 100644
> --- a/fs/tracefs/inode.c
> +++ b/fs/tracefs/inode.c
> @@ -21,6 +21,7 @@
> #include <linux/seq_file.h>
> #include <linux/magic.h>
> #include <linux/slab.h>
> +#include <linux/security.h>
>
> #define TRACEFS_DEFAULT_MODE 0700
>
> @@ -28,6 +29,23 @@ static struct vfsmount *tracefs_mount;
> static int tracefs_mount_count;
> static bool tracefs_registered;
>
> +static int default_open_file(struct inode *inode, struct file *filp)
> +{
> + struct dentry *dentry = filp->f_path.dentry;
> + struct file_operations *real_fops;
> + int ret;
> +
> + if (!dentry)
> + return -EINVAL;
> +
> + ret = security_locked_down(LOCKDOWN_TRACEFS);
> + if (ret)
> + return ret;
> +
> + real_fops = dentry->d_fsdata;

real_fops are NULL in my test case.

> + return real_fops->open(inode, filp);
> +}
> +
> static ssize_t default_read_file(struct file *file, char __user *buf,
> size_t count, loff_t *ppos)
> {
> @@ -210,6 +228,12 @@ static int tracefs_apply_options(struct super_block *sb)
> return 0;
> }
>
> +static void tracefs_destroy_inode(struct inode *inode)
> +{
> + if (S_ISREG(inode->i_mode))
> + kfree(inode->i_fop);
> +}
> +
> static int tracefs_reconfigure(struct fs_context *fc)
> {
> struct super_block *sb = fc->root->d_sb;
> @@ -236,6 +260,7 @@ static int tracefs_show_options(struct seq_file *m, struct dentry *root)
>
> static const struct super_operations tracefs_super_operations = {
> .statfs = simple_statfs,
> + .destroy_inode = tracefs_destroy_inode,
> .show_options = tracefs_show_options,
> };
>
> @@ -372,6 +397,7 @@ struct dentry *tracefs_create_file(const char *name, umode_t mode,
> struct dentry *parent, void *data,
> const struct file_operations *fops)
> {
> + struct file_operations *proxy_fops;
> struct dentry *dentry;
> struct inode *inode;
>
> @@ -387,8 +413,20 @@ struct dentry *tracefs_create_file(const char *name, umode_t mode,
> if (unlikely(!inode))
> return failed_creating(dentry);
>
> + proxy_fops = kzalloc(sizeof(struct file_operations), GFP_KERNEL);
> + if (unlikely(!proxy_fops)) {
> + iput(inode);
> + return failed_creating(dentry);
> + }
> +
> + if (!fops)
> + fops = &tracefs_file_operations;
> +
> + dentry->d_fsdata = (void *)fops;
> + memcpy(proxy_fops, fops, sizeof(*proxy_fops));
> + proxy_fops->open = default_open_file;
> inode->i_mode = mode;
> - inode->i_fop = fops ? fops : &tracefs_file_operations;
> + inode->i_fop = proxy_fops;
> inode->i_private = data;
> d_instantiate(dentry, inode);
> fsnotify_create(dentry->d_parent->d_inode, dentry);
> diff --git a/include/linux/security.h b/include/linux/security.h
> index d92323b44a3f..807dc0d24982 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -121,6 +121,7 @@ enum lockdown_reason {
> LOCKDOWN_KPROBES,
> LOCKDOWN_BPF_READ,
> LOCKDOWN_PERF,
> + LOCKDOWN_TRACEFS,
> LOCKDOWN_CONFIDENTIALITY_MAX,
> };
>
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 88064ce1c844..173191562047 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -36,6 +36,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
> [LOCKDOWN_KPROBES] = "use of kprobes",
> [LOCKDOWN_BPF_READ] = "use of bpf to read kernel RAM",
> [LOCKDOWN_PERF] = "unsafe use of perf",
> + [LOCKDOWN_TRACEFS] = "use of tracefs",
> [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
> };
>

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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