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Subject[PATCH] proc: fix a race in do_io_accounting()
There is a ptrace_may_access() check in do_io_accounting() to prevent
gathering information of setuid'ed and similar binaries. However, there
is a race against execve(). Holding task->signal->cred_guard_mutex
while gathering the information should protect against the race.

The order of locking is similar to the one inside of
ptrace_attach(): first goes cred_guard_mutex, then lock_task_sighand().

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Cc: stable@kernel.org
---
fs/proc/base.c | 10 ++++++++--
1 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 083a4f2..c605ee1 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2711,9 +2711,12 @@ static int do_io_accounting(struct task_struct *task, char *buffer, int whole)
{
struct task_io_accounting acct = task->ioac;
unsigned long flags;
+ int result = -EACCES;
+
+ mutex_lock(&task->signal->cred_guard_mutex);

if (!ptrace_may_access(task, PTRACE_MODE_READ))
- return -EACCES;
+ goto out_unlock;

if (whole && lock_task_sighand(task, &flags)) {
struct task_struct *t = task;
@@ -2724,7 +2727,7 @@ static int do_io_accounting(struct task_struct *task, char *buffer, int whole)

unlock_task_sighand(task, &flags);
}
- return sprintf(buffer,
+ result = sprintf(buffer,
"rchar: %llu\n"
"wchar: %llu\n"
"syscr: %llu\n"
@@ -2739,6 +2742,9 @@ static int do_io_accounting(struct task_struct *task, char *buffer, int whole)
(unsigned long long)acct.read_bytes,
(unsigned long long)acct.write_bytes,
(unsigned long long)acct.cancelled_write_bytes);
+out_unlock:
+ mutex_unlock(&task->signal->cred_guard_mutex);
+ return result;
}

static int proc_tid_io_accounting(struct task_struct *task, char *buffer)
--
1.7.0.4

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