[PATCH] psi: put NULL char correctly in psi_write()

[PATCH] psi: put NULL char correctly in psi_write()

[Miles Chen]

When passing a equal or more then 32 bytes long string to psi_write(),
psi_write() copies 31 bytes to its buf and overwrites buf[30]
with '\0'. Which makes the input string 1 byte shorter than
it should be.

Fix it by copying sizeof(buf) bytes when nbytes >= sizeof(buf).

This does not cause problems in normal use case like:
"some 500000 10000000" or "full 500000 10000000" because they
are less than 32 bytes in length.

	/* assuming nbytes == 35 */
	char buf[32];

	buf_size = min(nbytes, (sizeof(buf) - 1)); /* buf_size = 31 */
	if (copy_from_user(buf, user_buf, buf_size))
		return -EFAULT;

	buf[buf_size - 1] = '\0'; /* buf[30] = '\0' */

Before:
%cd /proc/pressure/
%echo "123456789|123456789|123456789|1234" > memory
[   22.473497] nbytes=35,buf_size=31
[   22.473775] 123456789|123456789|123456789| (print 30 chars)
%sh: write error: Invalid argument

%echo "123456789|123456789|123456789|1" > memory
[   64.916162] nbytes=32,buf_size=31
[   64.916331] 123456789|123456789|123456789| (print 30 chars)
%sh: write error: Invalid argument

After:
%cd /proc/pressure/
%echo "123456789|123456789|123456789|1234" > memory
[  254.837863] nbytes=35,buf_size=32
[  254.838541] 123456789|123456789|123456789|1 (print 31 chars)
%sh: write error: Invalid argument

%echo "123456789|123456789|123456789|1" > memory
[ 9965.714935] nbytes=32,buf_size=32
[ 9965.715096] 123456789|123456789|123456789|1 (print 31 chars)
%sh: write error: Invalid argument

Signed-off-by: Miles Chen <miles.chen@mediatek.com>
---
 kernel/sched/psi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 6e52b67b420e..517e3719027e 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -1198,7 +1198,7 @@ static ssize_t psi_write(struct file *file, const char __user *user_buf,
 	if (static_branch_likely(&psi_disabled))
 		return -EOPNOTSUPP;
 
-	buf_size = min(nbytes, (sizeof(buf) - 1));
+	buf_size = min(nbytes, sizeof(buf));
 	if (copy_from_user(buf, user_buf, buf_size))
 		return -EFAULT;
 
-- 
2.18.0

[tip: sched/core] sched/psi: Correct overly pessimistic size calculation

[tip-bot2 for Miles Chen]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     4adcdcea717cb2d8436bef00dd689aa5bc76f11b
Gitweb:        https://git.kernel.org/tip/4adcdcea717cb2d8436bef00dd689aa5bc76f11b
Author:        Miles Chen <miles.chen@mediatek.com>
AuthorDate:    Thu, 12 Sep 2019 18:34:52 +08:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Fri, 13 Sep 2019 07:49:28 +02:00

sched/psi: Correct overly pessimistic size calculation

When passing a equal or more then 32 bytes long string to psi_write(),
psi_write() copies 31 bytes to its buf and overwrites buf[30]
with '\0'. Which makes the input string 1 byte shorter than
it should be.

Fix it by copying sizeof(buf) bytes when nbytes >= sizeof(buf).

This does not cause problems in normal use case like:
"some 500000 10000000" or "full 500000 10000000" because they
are less than 32 bytes in length.

	/* assuming nbytes == 35 */
	char buf[32];

	buf_size = min(nbytes, (sizeof(buf) - 1)); /* buf_size = 31 */
	if (copy_from_user(buf, user_buf, buf_size))
		return -EFAULT;

	buf[buf_size - 1] = '\0'; /* buf[30] = '\0' */

Before:

 %cd /proc/pressure/
 %echo "123456789|123456789|123456789|1234" > memory
 [   22.473497] nbytes=35,buf_size=31
 [   22.473775] 123456789|123456789|123456789| (print 30 chars)
 %sh: write error: Invalid argument

 %echo "123456789|123456789|123456789|1" > memory
 [   64.916162] nbytes=32,buf_size=31
 [   64.916331] 123456789|123456789|123456789| (print 30 chars)
 %sh: write error: Invalid argument

After:

 %cd /proc/pressure/
 %echo "123456789|123456789|123456789|1234" > memory
 [  254.837863] nbytes=35,buf_size=32
 [  254.838541] 123456789|123456789|123456789|1 (print 31 chars)
 %sh: write error: Invalid argument

 %echo "123456789|123456789|123456789|1" > memory
 [ 9965.714935] nbytes=32,buf_size=32
 [ 9965.715096] 123456789|123456789|123456789|1 (print 31 chars)
 %sh: write error: Invalid argument

Also remove the superfluous parentheses.

Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Cc: <linux-mediatek@lists.infradead.org>
Cc: <wsd_upstream@mediatek.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190912103452.13281-1-miles.chen@mediatek.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/psi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 7acc632..4b14a32 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -1190,7 +1190,7 @@ static ssize_t psi_write(struct file *file, const char __user *user_buf,
 	if (static_branch_likely(&psi_disabled))
 		return -EOPNOTSUPP;
 
-	buf_size = min(nbytes, (sizeof(buf) - 1));
+	buf_size = min(nbytes, sizeof(buf));
 	if (copy_from_user(buf, user_buf, buf_size))
 		return -EFAULT;
 

RE: [tip: sched/core] sched/psi: Correct overly pessimistic size calculation

[David Laight]

From: Miles Chen
> Sent: 13 September 2019 23:43
> The following commit has been merged into the sched/core branch of tip:
> 
> Commit-ID:     4adcdcea717cb2d8436bef00dd689aa5bc76f11b
> Gitweb:        https://git.kernel.org/tip/4adcdcea717cb2d8436bef00dd689aa5bc76f11b
> Author:        Miles Chen <miles.chen@mediatek.com>
> AuthorDate:    Thu, 12 Sep 2019 18:34:52 +08:00
> Committer:     Ingo Molnar <mingo@kernel.org>
> CommitterDate: Fri, 13 Sep 2019 07:49:28 +02:00
> 
> sched/psi: Correct overly pessimistic size calculation
> 
> When passing a equal or more then 32 bytes long string to psi_write(),
> psi_write() copies 31 bytes to its buf and overwrites buf[30]
> with '\0'. Which makes the input string 1 byte shorter than
> it should be.
> 
> Fix it by copying sizeof(buf) bytes when nbytes >= sizeof(buf).
> 
> This does not cause problems in normal use case like:
> "some 500000 10000000" or "full 500000 10000000" because they
> are less than 32 bytes in length.
> 
> 	/* assuming nbytes == 35 */
> 	char buf[32];
> 
> 	buf_size = min(nbytes, (sizeof(buf) - 1)); /* buf_size = 31 */
> 	if (copy_from_user(buf, user_buf, buf_size))
> 		return -EFAULT;
> 
> 	buf[buf_size - 1] = '\0'; /* buf[30] = '\0' */

Wouldn't it be better to also allow the user to pass
an unterminated string?
So leave the '-1' on the assignment to buf_size, but remove
it from the last line.

	David

-
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