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Subject[PATCH RFC] ARM ETM driver: Do not deref potentially null pointer and don't allocate and free mem while holding lock.
Hello,

Looking at etb_read() in arch/arm/kernel/etm.c I noticed two things.

1. We are allocting and freeing 'buf' with vmalloc() while holding a
mutex locked. I cannot see any reason why we have to hold the mutex
just to allocate and free a bit of memory, so I moved it outside the
lock.

2. If the memory allocation fails we'll dereference a null pointer
further down since we never check the vmalloc() return value.
for (i = 0; i < length / 4; i++)
buf[i] = etb_readl(t, ETBR_READMEM);
The best way I could find to handle this was to simply return 0 when
we can't allocate memory, but there might be a better option that I
just couldn't find - in any case it is better than crashing the
kernel.

Please consider merging, but please also review carefully first since I'm
not familliar with this code.

CC me on replies please.


Signed-off-by: Jesper Juhl <jj@chaosbits.net>
---
etm.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
note: completely untested patch since I have neither hardware nor
toolchain to test it, so please review carefully and test before applying
it.

diff --git a/arch/arm/kernel/etm.c b/arch/arm/kernel/etm.c
index 11db628..30f845b 100644
--- a/arch/arm/kernel/etm.c
+++ b/arch/arm/kernel/etm.c
@@ -274,7 +274,10 @@ static ssize_t etb_read(struct file *file, char __user *data,
long length;
struct tracectx *t = file->private_data;
u32 first = 0;
- u32 *buf;
+ u32 *buf = vmalloc(length);
+
+ if (!buf)
+ return 0;

mutex_lock(&t->mutex);

@@ -292,7 +295,6 @@ static ssize_t etb_read(struct file *file, char __user *data,
etb_writel(t, first, ETBR_READADDR);

length = min(total * 4, (int)len);
- buf = vmalloc(length);

dev_dbg(t->dev, "ETB buffer length: %d\n", total);
dev_dbg(t->dev, "ETB status reg: %x\n", etb_readl(t, ETBR_STATUS));
@@ -310,10 +312,10 @@ static ssize_t etb_read(struct file *file, char __user *data,
etb_lock(t);

length -= copy_to_user(data, buf, length);
- vfree(buf);

out:
mutex_unlock(&t->mutex);
+ vfree(buf);

return length;
}


--
Jesper Juhl <jj@chaosbits.net> http://www.chaosbits.net/
Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html
Plain text mails only, please.



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