drm/i915/cmdparser: Check reg_table_count before derefencing.

Web:        https://git.kernel.org/torvalds/c/b18224e95cb13ef7517aa26e6b47c85117327f11
Commit:     b18224e95cb13ef7517aa26e6b47c85117327f11
Parent:     5e1df40f40ee45a97bb1066c3d71f0ae920a9672
Refname:    refs/heads/master
Author:     Michal Srb <[email protected]>
AuthorDate: Mon Feb 5 16:04:37 2018 +0000
Committer:  Rodrigo Vivi <[email protected]>
CommitDate: Tue Feb 6 11:41:39 2018 -0800

    drm/i915/cmdparser: Check reg_table_count before derefencing.
    
    The find_reg function was assuming that there is always at least one table in
    reg_tables. It is not always true.
    
    In case of VCS or VECS, the reg_tables is NULL and reg_table_count is 0,
    implying that no register-accessing commands are allowed. However, the command
    tables include commands such as MI_STORE_REGISTER_MEM. When trying to check
    such command, the find_reg would dereference NULL pointer.
    
    Now it will just return NULL meaning that the register was not found and the
    command will be rejected.
    
    Fixes: 76ff480ec963 ("drm/i915/cmdparser: Use binary search for faster register lookup")
    Signed-off-by: Michal Srb <[email protected]>
    Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
    Cc: Chris Wilson <[email protected]>
    Cc: Matthew Auld <[email protected]>
    Reviewed-by: Chris Wilson <[email protected]>
    Signed-off-by: Chris Wilson <[email protected]>
    Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
    register lookup")
    (cherry picked from commit 2f265fad9756a40c09e3f4dcc62d5d7fa73a9fb2)
    Signed-off-by: Rodrigo Vivi <[email protected]>
---
 drivers/gpu/drm/i915/i915_cmd_parser.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_cmd_parser.c b/drivers/gpu/drm/i915/i915_cmd_parser.c
index ccb5ba043b63..ab4c8b0ec886 100644
--- a/drivers/gpu/drm/i915/i915_cmd_parser.c
+++ b/drivers/gpu/drm/i915/i915_cmd_parser.c
@@ -1032,7 +1032,7 @@ find_reg(const struct intel_engine_cs *engine, bool is_master, u32 addr)
 	const struct drm_i915_reg_table *table = engine->reg_tables;
 	int count = engine->reg_table_count;
 
-	do {
+	for (; count > 0; ++table, --count) {
 		if (!table->master || is_master) {
 			const struct drm_i915_reg_descriptor *reg;
 
@@ -1040,7 +1040,7 @@ find_reg(const struct intel_engine_cs *engine, bool is_master, u32 addr)
 			if (reg != NULL)
 				return reg;
 		}
-	} while (table++, --count);
+	}
 
 	return NULL;
 }
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