Messages in this thread Patch in this message | | | From | Duncan Sands <> | Subject | [PATCH] bttv: correct bttv_risc_packed buffer size | Date | Wed, 25 Jan 2006 11:24:27 +0100 |
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This patch fixes the strange crashes I was seeing after using my bttv card to watch television. They were caused by a buffer overflow in bttv_risc_packed.
The instruction buffer size calculation contains two errors: (a) a non-zero padding value can push the start of the next bpl section to just before a page border, leading to more scanline splits and thus additional instructions. (b) the first DMA region can be smaller than one page, so there can be a scanline split even if bpl*lines is smaller than PAGE_SIZE.
For example, consider the case where offset is 0, bpl is 2, padding is 4094, lines is smaller than 2048, the first DMA region has size 1 and all others have size PAGE_SIZE, assumed to equal 4096. Then all bpl regions cross page borders and the number of instructions written is 2*lines+2, rather than lines+2 (the current estimate). With this patch the number of instructions for this example is estimated to be 2*lines+3.
Also, the BUG_ON that was supposed to catch buffer overflows contained a thinko causing it fire only if the buffer was overrun by a factor of 16 or more.
I didn't check whether similar mistakes exist elsewhere in the bttv code.
Signed-off-by: Duncan Sands <baldrick@free.fr>
PS: I'm sending the patch as an attachment because for some reason my mailer crashes if I try to insert it into the email. Index: Linux/drivers/media/video/bttv-risc.c =================================================================== --- Linux.orig/drivers/media/video/bttv-risc.c 2006-01-24 10:09:21.000000000 +0100 +++ Linux/drivers/media/video/bttv-risc.c 2006-01-24 10:16:06.000000000 +0100 @@ -51,8 +51,10 @@ int rc; /* estimate risc mem: worst case is one write per page border + - one write per scan line + sync + jump (all 2 dwords) */ - instructions = (bpl * lines) / PAGE_SIZE + lines; + one write per scan line + sync + jump (all 2 dwords). padding + can cause next bpl to start close to a page border. First DMA + region may be smaller than PAGE_SIZE */ + instructions = 1 + ((bpl + padding) * lines) / PAGE_SIZE + lines; instructions += 2; if ((rc = btcx_riscmem_alloc(btv->c.pci,risc,instructions*8)) < 0) return rc; @@ -104,7 +106,7 @@ /* save pointer to jmp instruction address */ risc->jmp = rp; - BUG_ON((risc->jmp - risc->cpu + 2) / 4 > risc->size); + BUG_ON(4 * (risc->jmp - risc->cpu + 2) > risc->size); return 0; } | |