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Message-ID: <3028ed35-3b6d-459f-f3c8-103c5636fe95@samsung.com>
Date: Tue, 13 Aug 2019 09:21:09 +0200
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: Matthew Garrett <matthewgarrett@...gle.com>, jmorris@...ei.org
Cc: linux-security-module@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
Matthew Garrett <mjg59@...gle.com>,
Steven Rostedt <rostedt@...dmis.org>,
Krzysztof Kozlowski <krzk@...nel.org>
Subject: Re: [PATCH V37 27/29] tracefs: Restrict tracefs when the kernel is
locked down
Hi again,
On 2019-08-13 08:10, Marek Szyprowski wrote:
> 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@...gle.com>
>> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.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.
Too much of a hurry. real_fops are okay in that test case...
>
>> + return real_fops->open(inode, filp);
... the issue is caused by NULL ->open() callback. Switching the above
line to:
return real_fops->open ? real_fops->open(inode, filp) : 0;
fixes the issue.
>> +}
>> +
>> 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
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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