linux-kernel - [PATCH 3.16 048/328] ALSA: memalloc: Don't exceed over the requested size

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Message-ID: <lsq.1544392233.171404831@decadent.org.uk>
Date:   Sun, 09 Dec 2018 21:50:33 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Takashi Iwai" <tiwai@...e.de>,
        "zhang jun" <jun.zhang@...el.com>
Subject: [PATCH 3.16 048/328] ALSA: memalloc: Don't exceed over the
 requested size

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@...e.de>

commit dfef01e150824b0e6da750cacda8958188d29aea upstream.

snd_dma_alloc_pages_fallback() tries to allocate pages again when the
allocation fails with reduced size.  But the first try actually
*increases* the size to power-of-two, which may give back a larger
chunk than the requested size.  This confuses the callers, e.g. sgbuf
assumes that the size is equal or less, and it may result in a bad
loop due to the underflow and eventually lead to Oops.

The code of this function seems incorrectly assuming the usage of
get_order().  We need to decrease at first, then align to
power-of-two.

Reported-and-tested-by: he, bo <bo.he@...el.com>
Reported-by: zhang jun <jun.zhang@...el.com>
Signed-off-by: Takashi Iwai <tiwai@...e.de>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 sound/core/memalloc.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

--- a/sound/core/memalloc.c
+++ b/sound/core/memalloc.c
@@ -239,16 +239,12 @@ int snd_dma_alloc_pages_fallback(int typ
 	int err;

 	while ((err = snd_dma_alloc_pages(type, device, size, dmab)) < 0) {
-		size_t aligned_size;
 		if (err != -ENOMEM)
 			return err;
 		if (size <= PAGE_SIZE)
 			return -ENOMEM;
-		aligned_size = PAGE_SIZE << get_order(size);
-		if (size != aligned_size)
-			size = aligned_size;
-		else
-			size >>= 1;
+		size >>= 1;
+		size = PAGE_SIZE << get_order(size);
 	}
 	if (! dmab->area)
 		return -ENOMEM;